Shunters are usually small in size, operate at very low speeds and therefore need a compact, high ratio box. With the motor revved up, running quality and power output are greatly improved as that high torque is geared right down.
For small-wheeled engines, a 60:1 RoadRunner fitted with a quality motor would be well up to the job, but if you want a loco that will really crawl, look at the higher ratio, double-reduction LoadHaulers.
If there's limited height available, the HumpShunter gives you same running qualities in an even smaller space, while for really tight confines, the super-compact MicroMiser and infinitley flexible LoLoader are the answer to many problems.
LNER Y4 - LoadHauler + with 1219 coreless motor
Straight gearbox makes best use of tank space.
Small wheels so 90:1 ratio will be very slow.
LNER Q1 - LoadHauler + with 1624 coreless motor
In OO gauge, tall box needed to lift motor above wheels.
Motor very slow and powerful so 68:1 ratio is ideal.
BR Class O1 DM - HumpShunter with 1020FE
Short box keeps visible cab space clear.
120:1 ratio for dead-slow shunting.