Abstract: There is disclosed a fixture for mounting an end of train marker light and/or other electronic apparatus to a railroad freight car coupler including an upright tubular member through which a threaded rod passes and extends into and through the flag hole in a standard railroad coupler; a frustro-conical enlargement at the bottom end of the tubular member seats in the top of the flag hole, and a frustro-conical collar threaded on the bottom of the rod is urged upward by rotating the rod to clamp the fixture in the flaghole. The top of the rod has a horizontal bar which is used to manually rotate the rod, and a keyed sleeve at the top of the tube may be raised to engage the bar to prevent its rotation while a padlock is secured through holes provided in the top of the sleeve.