You can actually run the IRT cars on BMT/IND tracks, it would just be awkward for passengers. Maintenance trains (e.g. the garbage collector train, the trains

You can actually run the IRT cars on BMT/IND tracks, it would just be awkward for passengers. Maintenance trains (e.g. the garbage collector train, the trains used for trackwork, etc.) often use retired IRT cars for things like motive power or transporting equipment for that very reason: they can go on any line.