To provide an expandable rail device for a railway working vehicle, quickly moving a movable rail with comparatively small driving force.

A push-up means 11 for pushing the tip part of a fixed rail 1 upward in moving backward a movable rail 2 is provided between the fixed rail 1 and the movable rail 1. The push-up means 11 has a push-up roller 12 and a push-up mechanism 13. The roller 12 is projected on a support plate 10 extending from the base end side of the movable rail 2. The push-up mechanism 13 engages with the push-up roller 12 to push up the movable rail 2 in moving backward the movable rail 2. The push-up mechanism 13 has a push-up lever 14 and an energizing means 15 for always energizing the lever 14 to turn in the erecting direction. In manually operating the movable rail 2 to be moved backward, the push-up roller 12 is pushed up through the push-up mechanism 13 by the energizing force of the energizing means 15, whereby sliding between the fixed rail 1 and the movable rail 2 is prevented.