Section 221.13 - Marking device display(a) During the periods prescribed in paragraph (b) of this section, each train to which this part applies that occupies or operates on main track shall(2) display on the trailing end of the rear car of that train, and(3) continuously illuminate or flash a marking device prescribed in this subpart.(b) Unless equipped with a functioning photoelectric cell activation mechanism complying with paragraph (c) of this section, the marking devices prescribed by this subpart shall be illuminated continuously or flash during the period between one hour before sunset and one hour after sunrise, and during all other hours when weather conditions so restrict visibility that the end silhouette of a standard box car cannot be seen from 1/2 mile on tangent track by a person having 20/20 corrected vision.(c) Marking devices prescribed by this part and equipped with a functioning photoelectric cell activation mechanism shall illuminate or flash the device continuously when there is less than 1.0 candela per square meter of ambient light.(d) The centroid of the marking device must be located above the coupler, where its visibility is not obscured and it does not interfere with an employee's access to, or use of, any other safety appliance on the car.51 FR 25185, July 10, 1986, as amended at 85 FR 80569 , Dec. 11, 2020 As amended at 85 FR 80569, 12/11/2020