The K4s quickly endeared themselves to the Scottish crews and, apart from some heavily loaded summer trains, eliminated uneconomic double-heading over the West Highland. However, as with all Gresley 2-6-0s it could be a rough ride at speed, and a locomotive designed to climb was not suited to the flat straight stretches of the line into Glasgow Queen Street railway station, or the 8.5 miles stretch alongside Loch Eil. Ride induced vibration was a problem on these stretches, and the middle big-end bearing would require regular nut tightening, with the middle connecting rod dropping off on one occasion: this resulted in increased maintenance inspections.