At a railroad crossing with no gates, warning signal, or flagman, within how many feet of the crossing does the 15 mph speed limit apply when you cannot see the tracks for 400 feet in both directions?

The 400-foot figure names the sight-line condition that turns the 15 mph cap on, not the distance the cap extends from the crossing itself. This answer picks the larger 400-foot number from the same sentence on the assumption the outer distance figure must define the outer edge of the reduced-speed zone.

Fifteen feet is the stop-back distance from the nearest track once a crossing device or person signals an oncoming train. This answer borrows the 15-foot stop-back distance that only applies once a warning device signals a train, or carries the 15 from 15 mph across to the distance answer.