49 CFR § 236.307
Indication locking
November 18, 2020
Indication locking shall be provided for operative approach signals of the semaphore type, power-operated home signals, power-operated switches, movable-point frogs and derails, and for all approach signals except light signals, all aspects of which are controlled by polar or coded track circuits or line circuits so arranged that a single fault will not permit a more favorable aspect than intented to be displayed.
[49 FR 3385, Jan. 26, 1984]