The superheated vertical water-tube boiler and the engine were similar to those used in Sentinel steam wagons. The engines had poppet valves and reversing was by sliding camshaft. The advantage of the water-tube boiler was that steam could be raised much more quickly than with a conventional fire-tube boiler.
Final drive to the wheels was by sprocket chain. Some locomotives had a sprocket gear ratio of 19:19 and others 15:19. In addition, there was a two-speed gearbox but gears could only be changed while the locomotive was stationary. Tractive effort was:
- Ratio 19:19, Low gear 12,600 lbf (56 kN), High gear 4,705 lbf (20.93 kN) (68154-68179 and 68184-68185)
- Ratio 15:19, Low gear 15,960 lbf (71.0 kN), High gear 5,960 lbf (26.5 kN) (68180-68183)
