A monomotor is a train

Train

A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

 design where a single motor

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 powers two axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

s in the same bogie

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

.

Conventional bogie design involves either having one motor for each axle, or having one or more axles unpowered. The design causes the motor to give both axles the same number of rounds per minute. When both axle's wheels are equally worn, this gives good operating conditions. But as soon as one of the wheels becomes slightly more worn (as is inevitable with steel on rail wheels), the one axle will perform all the work. This causes sliding and increases the wear. The problem can be countered by keeping the wheels regularly grinded to the same thickness.