The bogie axle connecting the wheels keeps them all rotating at the same speed. Bogies are classified by the number of axles they support. The axles are usually grouped by two per bogie, according to Vassilios A. Profillidis, author of "Railway Management and Engineering".
Each axle or pair of axles may have its own motor driving it through the gearbox. Some systems which are more sophisticated include axle boxes equipped with speed sensors, according to Railway-Technical.com.
The bogie axles and the car body serve as connection points for the shock absorbers and springs which support the car body, absorb vibration, help stabilize its ride, and minimize noise
