1) Passenger vehicle on tracks 2) Railcars 3) Transport that runs on tracks 
A railbus is a very lightweight type passenger rail vehicle (typically non-articulated or rigid frame) that shares many aspects of their construction with a bus, usually having a bus, or modified bus body, and having four wheels on a fixed base, instead of on bogies. These railbuses, a design developed in the 1930s, have evolved into larger dimens...
A DMU the size of a bus, sometimes made using bus components.