My version of the British Pacer class 14x diesel railcar/railbus. Pacers or nodding donkeys were made in 1980s using bus bodies on 2 axle rail frames. The design provides a rough ride over switch points and rail joints. It also makes much squealing noise going through curves.
Pacer Railcar by dr_spock_888, on Flickr
In keeping with bus parts and keeping costs down as with the real Pacers, one of the windscreen came off a LEGO City Bus.
Interior seating for 4 minifig passengers. There is a passthrough between cars.
Video of Pacer motorized it with Power Functions.