The exchange of cars between two different railroads. The location where cars are exchanged between two different railroads. Cars are interchanged when they are placed on the interchange track or when they pass an interchange point (in the case of run through trains).
If the connection is between two routes of the same railroad, it technically is not an interchange, its a junction because the cars do not change hands between railroads.
Not all locations where railroads cross or join are interchanges. A book called the “Open and Prepay” lists valid interchange locations. Interchange locations are also listed in each railroad’s entry in the Official Railway Equipment Register (ORER).