From Railroad Depot

The commutator is the slotted copper segments at the end of the armature on an electric motor, which transfers the current from the brushes to the coils wound on the armature. In a model locomotive, the two brushes are connected to opposite rails to transfer the track power to the motor. A spring under the brush both forces the brush onto the commutator, as well as carrying power from the track to the brush.

[edit] Care and Cleaning

Most model railroaders are diligent about keeping their locomotive wheels clean, however, many forget to make sure the commutator is kept clean. A dirty commutator can result it sluggish or even no operation, and a lot of frustration. Although the commutator will not need to be cleaned as often as the wheels, it's a good idea to inspect this part of the motor at least once a year.

Most locomotives will allow you to clean the commutator without removing the motor, however, you will need to remove the shell of the locomotive to gain access to the motor. A small drop of lubricating oil or track cleaning solution on the commutator and a quick run of the locomotive will generally be enough to restore full operation.

[edit] Replacement

Generally, the commutator in your model locomotive will never need to be replaced so long as you keep up it clean. In most cases, shoul dthe commutator need to be replaced in smaller scales, such as N Scale or HO scale, it is generally easier and cheaper to replace the entire motor. It's always a good idea to replace your motor when necessary with a good quality can motor or an identical replacement part. Less expensive motors may cause other problems and decrease the locomotives operating qualities.