Bogies

Train wheels are typically not part of the train itself, but standalone "bogies" that the train cars sit on, held in place by gravity.  The bogie, also known as a railroad truck, wheel truck, or simply truck in North America, bears the load, transmits the traction and braking forces, steers the car in a safe manner (e.g. cars can pivot) and smooths out track irregularities (i.e. springs). Bogies are also handy when changing between different track gauges.  The train cars are lifted up and one set of bogies is exchanged for another.

https://www.youtube.com/watch?v=6K4gQNfrR2A