49 CFR 214.339: Audible warning from trains.

(a) Each railroad shall have in effect and comply with written procedures that prescribe effective requirements for audible warning by horn and/or bell for trains and locomotives approaching any roadway workers or roadway maintenance machines that are either on the track on which the movement is occurring, or about the track if the roadway workers or roadway maintenance machines are at risk of fouling the track. At a minimum, such written procedures shall address:

(1) Initial horn warning;

(2) Subsequent warning(s); and

(3) Alternative warnings in areas where sounding the horn adversely affects roadway workers (e.g., in tunnels and terminals).

(b) Such audible warning shall not substitute for on-track safety procedures prescribed in this part.