This paper proposes a new rail vehicle concept: a 'three-axles-one-unit' vehicle, with a self-steering single axle. Usually, a single-axle vehicle requires a short and wide car body, which is a system design of the next generation of railway vehicles. The authors propose a new 'three-axle-one-unit' vehicle steering a single-axle truck for the railways. This proposed vehicle uses three trucks and has the standard body length of a conventional train. It has the merit of decreasing elastic-body vibration by using a lightweight car body and by setting a truck in the middle of the car body. This system design can provide high performance with respect to stability, curving performance, and axle load limitation. Analyses were made with multi-body dynamic simulations and the basic characteristics were cleared. Finally, 1/10-scale model was made and tested. The proposed vehicle's potential for high performance was proved from running experiments, including curving.