Historically, the rail gauge has had considerable variations, with narrow gauge common in many early systems. However, most light rail systems are now standard gauge. An important advantage of standard gauge is that standard railway maintenance equipment can be used on it, rather than custom-built machinery. Using standard gauge also allows light rail vehicles to be delivered and relocated conveniently using freight railways and locomotives. Additionally, more manufacturers are able to produce standard gauge vehicles, reducing costs for rolling stock acquisition. Another factor favoring standard gauge is that low-floor vehicles are becoming popular, and there is generally insufficient space for wheelchairs to move between the wheels in a narrow gauge layout.