FRA Regulation 49 CFR § 229.125(d)

(d) Effective December 31, 1997, each lead locomotive operated at a speed greater than 20 miles per hour over one or more public highway-rail crossings shall be equipped with operative auxiliary lights, in addition to the headlight required by paragraph (a) or (b) of this section. A locomotive equipped on March 6, 1996 with auxiliary lights in conformance with § 229.133 shall be deemed to conform to this section until March 6, 2000. All locomotives in compliance with § 229.133(c) shall be deemed to conform to this section. Auxiliary lights shall be composed as follows:

(1) Two white auxiliary lights shall be placed at the front of the locomotive to form a triangle with the headlight.
(i) The auxiliary lights shall be at least 36 inches above the top of the rail, except on MU locomotives and control cab locomotives where such placement would compromise the integrity of the car body or be otherwise impractical. Auxiliary lights on such MU locomotives and control cab locomotives shall be at least 24 inches above the top of the rail.
(ii) The auxiliary lights shall be spaced at least 36 inches apart if the vertical distance from the headlight to the horizontal axis of the auxiliary lights is 60 inches or more.
(iii) The auxiliary lights shall be spaced at least 60 inches apart if the vertical distance from the headlight to the horizontal axis of the auxiliary lights is less than 60 inches.

(2) Each auxiliary light shall produce a peak intensity of at least 200,000 candela
or shall produce at least 3,000 candela at an angle of 7.5 degrees and at least 400 candela at an angle of 20 degrees from the centerline of the locomotive when the light is aimed parallel to the tracks. Any of the following lamps meet the standard set forth in
this paragraph (d)(2): an operative PAR–56, 200-watt, 30-volt lamp; an operative PAR–56, 350-watt, 75-volt lamp; or an operative lamp of equivalent design and intensity.

(3) The auxiliary lights shall be focused horizontally within 15 degrees of the longitudinal centerline of the locomotive.

(e) Auxiliary lights required by paragraph (d) of this section may be arranged
(1) To burn steadily or
(2) Flash on approach to a crossing. If the auxiliary lights are arranged to flash;
(i) They shall flash alternately at a rate of at least 40 flashes per minute and at most 180 flashes per minute,
(ii) The railroad’s operating rules shall set a standard procedure for use of flashing lights at public highway rail grade crossings, and
(iii) The flashing feature may be activated automatically, but shall be capable
of manual activation and deactivation by the locomotive engineer.

(f) Auxiliary lights required by paragraph (d) of this section shall be continuously
illuminated immediately prior to and during movement of the locomotive, except as provided by railroad operating rules, timetable or special instructions, unless such exception
is disapproved by FRA. A railroad may except use of auxiliary lights at a specific
public highway-rail grade crossing by designating that exception in the railroad’s operating rules, timetable, or a special order. Any exception from use of auxiliary lights at a specific public grade crossing can be disapproved for a stated cause by FRA’s Associate Administrator for Safety or any one of FRA’s Regional Administrators, after investigation by FRA and opportunity for response from the railroad.