Monomotor bogie

This increased their unsprung weight and limited their maximum speed, despite the greater power of electric locomotives.

To achieve a higher speed, locomotives began using wheel arrangements such as 2'Do2' (AAR: 2-Do-2), with the SNCF Class 2D2 5500[1] This used leading (and trailing) pony trucks or bogies to provide stability at high speed, with individual traction motors for each axle, avoiding the weight of the coupling rods.

Previous French Bo-Bo locomotives had been considered suitable only for medium speeds, with the pony truck and rigid frame used for express passenger service.

The Swiss Re 4/4I of 1946 achieved speeds of up to 125 kilometres per hour (78 mph), owing to the advanced design of its bogies.

[4] Class CC 40100 (1964), a large quad-voltage locomotive, requiring a C'C' layout with three axle bogies.