How far before a railroad crossing must a bus driver come to a stop?

49 CFR 383.117(a)(4), Proper procedures at railroad-highway grade crossings and drawbridges

Stopping too close to the tracks can put the front of the bus in the path of a train or block the driver's view down the tracks; stopping too far back wastes time and visibility. The 15-to-50-foot window is the tested, specific range examiners expect drivers to know.