running board

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A narrow footboard extending under and beside the doors of some automobiles and other conveyances.

Century Dictionary and Cyclopedia

1. n. In electricity, a mounting for reels of wire by means of which, in the construction of overhead lines, several wires can be run off the reels simultaneously and fastened to the cross-arms of the poles.

Wiktionary

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

GNU Webster's 1913

1. adj. a narrow platform extending along the side of a locomotive.

WordNet 3.0

1. n. a narrow footboard serving as a step beneath the doors of some old cars