running board

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A narrow footboard extending under and beside the doors of some automobiles and other conveyances.

from The Century Dictionary.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a narrow footboard serving as a step beneath the doors of some old cars