Could Bus Multiple Units (BMU)s bridge the bus-rail divide?

Here at Portland Transport, we (both editors and commenters) frequently like to engage in a bit of technical speculation, hoping for future improvements that will allow transit agencies to do more with less.  There’s lots of talk around here about electric buses, of driverless vehicles, of different vehicle configurations, and even more exotic concepts like Personal Rapid Transit (PRT) and bus/train hybrids.   And it’s a tradition ’round these parts to announce groundbreaking new transit technologies the day following March 31st.  :)

We also discuss the merits of bus vs rail a lot, and the various types thereof:  Local bus vs various grades of Bus Rapid Transit.  Streetcar vs light rail vs heavy rail (high-platform long-consist trains found in many large-city subway systems) vs commuter rail.  Some of these debates can get spirited.

Today, I’m going to discuss some utterly speculative technology that might help bridge the operational gap between large rubber-tired passenger-hauling vehicles running on paved roads (“bus”) and steel-wheeled vehicles running on steel rails.  Since I’m not aware of any existing, well-used name for the technology I’m about to discuss, I shall call it a Bus Multiple Unit (BMU).

More after the jump:

Advantages of each mode

Much has been written about bus vs rail, and we do not intend to rehash that here.  Things that are orthogonal to mode (amenities, frequency, exclusiveness of right-of-way) we will not discuss, nor will we consider any “culture” attributes of either bus or rail.   Some advantages of bus (specifically BRT) are given here; and they can be summarized into two broad categories:  a) Lower construction costs (almost none for local bus that uses an existing street network), and b) far greater operational flexibility (not tied to a fixed guideway). This operational flexbility allows things like phased construction, easier mixing of express and local services, branching topologies and “open BRT”, easier maintenance, and easier routing around incidents.

The main advantage of rail (unfortunately, I don’t seem to have a companion piece to the BRT piece listed above) are a) a few minor improvements in comfort, energy efficiency, and vehicle reliability owing to the physics of steel on steel, and b) the ability to couple railcars into trains.  The latter is by far the most important advantage of rail–if you want to haul twenty thousand persons per hour down a corridor of any length, buses won’t cut it.  Even if the drivers worked for free, the number of (independent) buses would overwhelm the system.  But when a single six-car subway train can hold close to a thousand passengers, and these can come every two minutes, this sort of passenger load is quite practical with rail.

In the Portland context, 20000 passengers per hour per direction is tremendous overkill, obviously; the main MAX trunk line does about a fifth of that.  Two-car MAX and WES trains and one-car Streetcars don’t take all that much advantage of this capability of rail.   But this discussion is not intended to be Portland-specific, and many systems worldwide do exploit rail’s capability to move large numbers of people.

So what is a BMU?

What, then,  is a BMU?  It’s essentially similar to a rail DMU (diesel multiple unit) or EMU (electric multiple unit), though with rubber tires and which runs on pavement.  (This discussion ignores the propulsion technology employed).  A multiple-unit (MU) railcar is a railcar which contains a full set of propulsion and braking systems, and can either run as an independent vehicle, or can be joined into trains.  In the latter configuration, its propulsion and braking systems are coupled together and placed under the control of a single operator.  WES uses DMUs (though some of the WES cars from Colorado Railcar lack engines); all MAX vehicles are EMUs.  The Streetcars are also capable of EMU operation, but are never so operated in the Portland Streetcar system.  The multiple unit railcar stands in contrast to locomotive-hauled trains, where one or more dedicated engine cars provide traction (more than a single locomotive is seldom necessary for passenger trains), and unpowered coaches carry  passengers (and numerous other types of unpowered cars can haul freight).

The BMU is essentially the same concept, applied to buses:  Two or more buses are coupled–possibly mechanically, possibly just electrically–into a train, and the operator of the lead bus drives them all in tandem.  Each bus in the consist has its own traction, steering (necessary for unguided operation), and braking, and could possibly be driven as an independent vehicle.  (Headless buses, intended only for coupling into trains but without an operator’s cab, are also possible).  This stands in contrast to how existing multiple-trailer trucks operate, which is similar to the locomotive-coach model:  A tractor provides traction, and it tows up to three (in Oregon) unpowered trailers.  (In some remote parts of the world, such as Western Australia, so-called “road trains” can get far longer than what is street-legal anywhere in the US).

Why bother?

Both locomotive-hauled railcars and “multiple unit” railcars (or single-car trams) are very old technology.  The fixed guideway provided by rails makes locomotive-powered configurations stable.  Engines can even push smaller consists safely (this is common on commuter rail lines, though certainly not for large freight trains). And the lack of a need to steer makes multiple-unit synchronization easy–only acceleration and breaking need to be coupled, which can happen more or less simultaneously.  The choice of locomotive-hauled vs MU-based trains in passenger operations is generally made based on the consist size–locomotives are more efficient when hauling large numbers of coaches, but are overkill for small consists.

But joining rubber-tired vehicles into trains is another matter.  The lack of a fixed guideway makes both single-powerplant and distributed-power trains difficult.  Single-powerplant configurations are commonly found in freight operations:  the tractor-trailer rig.  But tractor-trailers tend to be unstable and difficult to handle, with stability getting worse as more trailers are added.  Steering them is difficult, particularly in tight spaces, and the trailers tend to sway when being pulled at high speed.  Driving them in reverse also can be difficult.  Such configurations are generally only allowed for freight, and trains longer than three trailers are illegal in the US (and triples only legal on designated routes, and in a few states).

Articulated buses do exist, of course, but these are generally only powered in one section (sometimes the back section is powered, sometimes the middle set of wheels).  Bi-articulated buses (with three sections) are found in some countries, though usually only on dedicated rights-of-way; many other countries ban them.  (Even the longest bus in service that I’m aware of has less capacity than a single MAX car).A “multiple unit” configuration of buses or trucks would alleviate the stability problems associated with hauling trailers.   But until recently, such things have been technologically infeasible; even today, the relevant technology is in the research stage.

Current research

There are several different research programs which are of interest:

  • A prototype automated highway (a highway in which cars would be coupled in platoons and driven by computer control rather than humans, to permit much denser spacing of vehicles and less chance of accident) system was demonstrated prior to the turn of the century, in 1997.  This system, which required both modified highway infrastructure and modified vehicles, is an arguably more difficult problem than the BMU as described here–it needs to support arbitrary and dynamic configurations of vehicles (potentially different makes and models, owned by different people, which enter and leave the system at various and unpredictable times), rather than static consists of similar vehicles operated by the same agency.  Much of the research in automated highways has slowed down, as self-driving vehicles may be a better technological choice for passenger cars and taxis.
  • More recently, a European research program called Safe Road Trains for the Environment(SARTE) is studying the problem of platooning trucks, so the driver can safely navigate a fleet of trucks down a highway.  This project has had several successful trials, though at this point is not ready for production.  It is oriented towards freight applications, not transit, though the design goals of this project are similar to what might be necessary to build BMUs.
  • Some transit agencies actually operate manual bus platoons–buses that are scheduled to operate in tandem, but each one with its own driver. This isn’t commonly done–below a certain operating frequency you’re better off staggering bus runs to minimize headways–but in some cases, it can be useful, as it replaces uncontrolled and unpredictable bunching (which degrades service) with controlled bunching.
  • One other relevant technology is the guided busway–which means to allow buses to operate part of the time on the regular street network, and part of the time in a fixed guideway, in order to achieve (for part of the journey, at least) some of the advantages of rail.  The Adelaide O-Bahn is one example of this–buses entering the busway are mechanically coupled to a roadside track, and can operate the busway at high speed without need to steer.  This is mainly used to permit fast operation in a narrow right-of-way and to allow precise platform docking; this is not used to allow buses to be entrained.

How would this play in Portland?

Obviously, the purpose of all of this is to allow buses to a) retain their operational flexibility, at least somewhat, but b) haul larger number of passengers per operator, and be capable of handling large passenger loads, by coupling buses into trains for at least part of the journey.  It is unlikely that “bus trains” will be street-legal for operation on arbitrary public streets; even a two-bus platoon would be over 80′ long–about the length of a triple-trailer.  (Street-legal articulated buses are up to 67′ long).  So use of BMUs would be a BRT-focused operation.  But even within the context of a dedicated BRT line (such as the Southwest Corridor), a BMU platoon could offer the following advantages over light rail, while maintaining similar passenger capacity per driver.

  • Non-platooned buses could still use the busway, and it would still be far easier to support skip-stop or express service than with rail.
  • At certain stations, buses could leave the platoon and proceed to different destinations.  In the opposite direction, buses arriving at a transit center could assemble into a platoon, with one driver continuing to downtown, and the others waiting for a return platoon to split up.  This would pose quite a bit of logistical challenges, but might be preferable to riders than having to transfer from feeder buses onto a trunk line.
  • Busways are less susceptible to disruption due to incidents than are rail lines, as buses can steer around obstacles.  Even if an incident blocks a busway completely, it may be possible to certify parallel roadways for emergency bus-platoon operation, allowing platoons to navigate around closures of the primary route.
  • Bus platoons travelling on the mall could be treated like MAX trains, stopping at the MAX platforms.
  • During non-peak hours, the platoon size can be more easily reduced.  While a four-bus platoon (the largest you’ll probably get once inter-bus spacing is factored in; not sure five-bus platoons can safely be accommodated downtown) is appropriate for peak travel times, there’s no need to operate these at night.  With BMUs, especially if not physically coupled, the logistics of shrinking or growing the platoon size would likely be simpler than shrinking or growing a MAX train.  (Type 4 and 5 MAX vehicles come in pairs and cannot be run as singles; and Type 1 LRTs have to be coupled with Type 2 or Type 3 for ADA compliance).

BMU platoons probably wouldn’t make sense for the Powell/Division line, both because much more of this will likely be street-running rather than in a separate right-of-way, and the technology will probably not be ready in time for a desired opening by 2020 or so.



21 Responses to Could Bus Multiple Units (BMU)s bridge the bus-rail divide?

  1. JV
    December 17, 2014 at 7:34 am Link

    Did you mean something like this ? In several ex-USSR towns, pairs of ETBs are/were operating in mixed traffic. They used Skoda 9Tr, Ziu9, experimentally also Skoda 14Tr and possibly other types.

  2. Jack G.
    December 17, 2014 at 7:32 pm Link

    I guess I don’t see how this system would provide benefits over either BRT/LRT w/ local buses, or “Open” BRT. On the Southwest Corridor for example, most of the bus lines that run from downtown along the corridor, turn off of Barbur either at Capital Hwy, or near Terwilliger/Bertha. If these buses were to be formed into BMU trains it would mean that there would need to be some area(s) for them to get formed up, and for the extra operators to wait for a new bus. All of this just 3 miles from the downtown transit mall.

    Then there’s also the issue of a bus being late. Would the bus miss it’s train? Would the entire train be late? What if the bus that’s late is the one that has the operator who is scheduled to take the combined train into downtown? I suppose if all the buses operated autonomously it could work, but what benefits would it provide to the riders that a dedicated RoW trunk line of autonomous buses couldn’t?

    • EngineerScotty
      December 17, 2014 at 10:08 pm Link

      Good questions, and what the post is trying to explore.

      At a minimum, this could provide LRT-equivalent capacities for similar labor costs, but with the option to detour around problems on the line. The idea of breaking apart consists mid-route (vs transfers or separate buses) is a more radical proposal, obviously, and with potential logistical complications, as you note.

      The economics of platooning are probably more significant for freight-hauling, which is a good reason why that’s where much of the research is being done.

      Of course, if you have fully autonomous buses that need zero drivers, that eliminates a major advantage of forming trains or platoons. But I expect that’s a tougher problem to solve.

  3. Anandakos
    December 18, 2014 at 10:08 am Link

    Since at this point it’s pretty clear that this would only be implemented in the Southwest Corridor, why not take the easy way out and go with “No Build”? Tualatin and Tigard don’t want The Invasion of the Body Haulers, and there’s not enough development potential along Barbur to make it worthwhile as a stub. So stick with what’s there today.

    In the immortal words of the Queen of France when questioned about congestion in the bread lines, “Let them sit still”. At least they can deep their taxes, non?

    • Jack G.
      December 18, 2014 at 10:27 am Link

      At the Southwest Corridor steering committee meeting, there was a lot of support for the project. Just because a narrow vote during an off-year election was pushed through, doesn’t mean that all the residents of Tigard are opposed to the project.

      • EngineerScotty
        December 18, 2014 at 10:54 am Link

        I don’t object to a public vote on this; but for goodness sake, have it at a regular election.

        That said, it should be clear that the options on the table are “SW Corridor” or “nothing”; not “SW Corridor” or “widen I-5 and OR-217” or “westside bypass”. Neither Portland nor Beaverton want wider freeways, and bulldozing prime Washington County farmland south and west of Cooper Mountain so folks living in Tualatin can get to jobs in Hillsboro faster, ought to be a non-starter.

      • SEAN
        December 18, 2014 at 6:42 pm Link


        If that be the case – then why didn’t these voters turn out in great numbers to drown out the NIMBY’s?

        On this interesting bus idea, what happens if a convoy of these busses needs to turn corners? I mean the radius for such a turn would be extremely tight. I’ve seen what happens when an articulated bus makes a turn & in some situations it requires the driver to have nearly perfect skills to pull it off. Try that with a linked convoy.

        • EngineerScotty
          December 18, 2014 at 9:07 pm Link

          The idea is that each bus would have independent steering and be able to navigate the turn as though they were a single uncoupled vehicle.

  4. Nathanael
    December 19, 2014 at 2:11 am Link

    Basically, the answer is “no”.

    The fundamental problem is fishtailing. There’s essentially one cheap way to fix that, and it’s called “rails”.

    You can build a “guided busway” (big concrete rails and inefficient rubber tires). Or you can just build steel rails and use steel wheels (much more efficient, much cheaper).

    People have been trying this sort of magical thinking for generations imagining that some sort of cargo-cult “technology” will be better than the highly-functional rails.

    You can’t beat the laws of physics. Two metal rails with conical metal wheels is:
    (a) passively self-stabilizing
    (b) extremely energy-efficient
    And as a result you can run long trains. Cheaply.

    There is no point in trying to reinvent this. None. Every time someone has tried, they have ended up inventing something more expensive to operate, and they’ve been trying for over 100 years.

    The latest fad is spending vast amounts of money on computers, sensors, etc. to try to do active stabilization… when passive stabilization is a solved problem and much, much cheaper. Use two rails, with conical wheels in between them.

    • Nathanael
      December 19, 2014 at 2:16 am Link

      If you want to try something halfway between bus and train, try this thing they’re building in Japan:

      This is based on tried-and-true tech and gets you the actual advantages of a train (coupling, passive stabilization). I’m not sure what happened to it after its initial launch…

    • EngineerScotty
      December 19, 2014 at 11:33 am Link

      The problem with passive stabilization vs active–is that it requires a guideway; meaning the vehicle(s) cannot go where the guideway isn’t. The whole point of active stabilization is that it doesn’t require a guideway. Why are guideways problematic? They’re expensive; switching/routing is a pain in the butt. There’s a reason that most rail transit systems lack redundancy and are prone to systemic failure: adding redundancy is expensive. (A legitimate criticism of MAX is that it was built on the cheap, with inadequate numbers of crossovers and sidings, a lack of proper reverse-direction signalling, and an inadequately-protected power supply; but were TriMet to budget for these things and add it to the plan, the sharp-pencils would descend like wolves and accuse the agency of wasting money on gold-plating).

      Technology does seem to be at least close to where practical solution to active stabilization is possible. While self-driving cars are perhaps overhyped–my suspicion is that we’re at least a decade out if not more from having such things in production–the problem addressed here is an easier one to solve.

      Perhaps ’tis a pipe dream–the demonstrated prototypes are operating in too controlled of an environment, and we won’t be able to do this at production scale for a long time. Nobody has announced any delivery schedules for this sort of thing.

      On the other hand, a nexus of greatly improved computer horsepower, advances in image processing, cheap digital cameras/sensors, and such–has made things possible that would have seemed like science fiction a decade or two ago.

      As noted at the top, this article is speculative. Until some vendor is willing to commit to a production schedule, it would be folly to plan a transit line (or system) around technology that does not yet exist.

      • Alexander Craghead
        December 19, 2014 at 4:30 pm Link

        MAX built on the cheap, because lack of crossovers…? Running light rail as dedicated double track (directional tracks, no opposing movements on a single track) is the norm in the U.S., not the exception.

        As for the BMU and getting rid of a fixed guideway… WHY? The number one reason any transit vehicle is late is because of traffic disruption. We should be working towards established dedicated rights of way and fixed guideways for as much as humanly possible, regardless of train or bus or whatever technology of propulsion.

        Get transit out of traffic, not more into it.

      • Nathanael
        December 29, 2014 at 1:12 pm Link

        Active stabilization is fundamentally more expensive than passive stabilization. A *lot* more expensive.

        It’s not that you can’t do it. You can do it, right now. It’s that it’s *grossly uneconomical*.

        The train is cheaper.

        This is what gadgetbahn fantasists and road nuts don’t understand. *The train is cheaper than your fantasy design and it will always be cheaper*. You can’t beat the laws of physics.

        • Nathanael
          December 29, 2014 at 1:14 pm Link

          (I should be more specific. What is the train cheaper for? It is cheaper for TRAINS — long convoys of vehicles. Not for single unit streetcars.)

  5. Lenny Anderson
    December 19, 2014 at 10:06 am Link

    For this to work, you’ve got to have an exclusive transit right-of-way. If you have that, why not just go with rails? 10 minute BRT service that splits at one end into three lines, just gives you lousy 30 minute service at the far end. Better a transfer to 15 minute local service.
    Of course we need to have a region-wide election for the next phase of transit projects. It should include SW Corridor HCT, extension of Yellow to Hayden Island, of Red to somewhere out there is Wash. county and Streetcar to Hollywood! It would be a property tax bond issue for local match.

  6. Wells
    December 19, 2014 at 10:29 am Link

    Self-driving autonomous vehicle idea is utter nonsense. Moreover, our senseless rush hour commuting only increases beyond both road and transit capacity when they’re by all means dedicated to long-distance commuting. Longer, uncomfortably overloaded LRT trains and buses during rush hours only leads more people driving at the same time. And off rush hours, LRT and buses run less than 1/4 full. There is some hope that the 2040 Regional Plan can change our dire situation, but not while such ridiculous ideas continue to be promoted. If BART station areas redeveloped with mixed-use infill, its 10-car rush hour trainsets could be reduced to 4-car trainsets all day.

    The next new bus technology actually needed are low-floor plug-in hybrid paratransit vans. Some form of these buses could replace many of the 40′ roaring, fuming rattletrap albatrosses now considered the standard.

  7. was carless
    December 23, 2014 at 2:27 pm Link

    Whatever happened to bendy-buses? They are much more mechanically simpler.

    • Dave
      December 23, 2014 at 3:15 pm Link

      Trimet refuses to buy them since they had a bad experience with them in the past. Instead they claim there is no need for them since Max is available in most of the areas where the ridership could justify it. I don’t agree with it as I can think of several routes that could use it.

      • SEAN
        December 31, 2014 at 11:31 am Link

        Artix have changed since the first generation in the 1980’s. The real issue with them relates to turning radius witch isn’t a big deal in most places. Portland has the unique problem of 200 X 200 blocks in the city core. It maybe a challenge for TriMet to operate them, but it’s not Impossible. My solution for that is to get Alexander Dennis double deckers. These busses achieve the ridership goals for artix, but in a standerd 40-feet. They do come in 42 & 45 foot lengths as well. has 150 of these busses in the fleet. They started with 50 of them in 2005 & were so successful that they kept adding more to where they are today.

        • Anandakos
          December 31, 2014 at 5:40 pm Link

          Double talls are great for commuter work, but suck for all-day on-off service. Since MAX handles the long-distance commuter express jobs for Portland, there’s less use for any high-capacity buses. That’s the primary reason that Tri-Met hasn’t used artics.

          That’s not to say there aren’t any routes that might use high capacity buses, but if they’re for all-day use, they’d be better as articulateds.

          • SEAN
            January 1, 2015 at 8:47 am Link

            Happy new year!

            I see your point, but there’s an additional element I neglected to mention & I know you’ll get it. The double talls bring a level of fascination & interest that an articulated bus just doesn’t bring. And that could attract new ridership. This is why I linked to the RTC in Las Vegas above.

            Not all Double talls operate on the “Strip”. Infact a high percentage of them travel on routes to & from suburban areas & it has atracted new riders despite the car oriented nature of the valley.

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