running board

noun2

1. •
   A narrow board serving as a step beneath the doors of some cars.

   • One of the "locomotors", a 72 h.p. machine for light yard shunting which can be driven by a shunter from the running board. [A long step on the side just above rail level] — 1962
2. •
   A narrow footway above the driving wheels of some steam locomotives, and along the sides of some diesel locomotives.