Mixed-traffic Locos with wheels up to around 6ft diameter, need to be capable of hauling a reasonable load at anything up to 60mph or so and, for this kind of application, the flexible RoadRunner series is perfect,
They're available with a choice of ratios that allow you to match the wheel size to track speed, to suit anything from a plodding goods to a nippy, local passenger.
Medium-sized tender engines often have small wheels and high boilers and can offer few options for concealing the mechanism. In these instances, a super-compact RoadRunner will enable you to fit a vertical motor into the limited firebox space. When more room is available, 34 or 45:1 ratios are ideally suited to the lower RPM and high torque from a larger motor.
BR Class 2 Tank - RoadRunner + with 1230 coreless motor
L-shaped box keeps cab clear.
Small wheels so 45:1 ratio selected for reasonable top speed.
GWR 43XX - RoadRunner+ with 1320 coreless motor
Vertical motor avoids cutting through boiler.
60:1 ratio chosen for low speeds on terminus layout.
Churchward 4-6-0 on OO guage - HiFlier with 16mm coreless motor
Tall box lifts the motor clear of the wheel flanges.