A bogie or railroad truck is a structure underneath a train to which wheel axles (and, hence, wheels) are attached. If they are used there are two for each carriage, wagon and locomotive, or alternatively, they are at the connections between the carriages or wagons. The connections of the bogies with the cars allow a small rotational movement around a vertical axis.
Usually the train floor is at a level above the bogies. However, for a double decker this applies only at the bogies: between them the bottom deck is lower.
There are low-floor trams without bogies.
- To be explained: notations such as 3-4-3, Bo-Bo and Co-Co.