49 CFR §246.107

1. (a)A railroad may classify its certified signal employees in occupational categories or subcategories by class, task, location, or other suitable terminology, in accordance with an FRA-approved certification program that complies with the requirements of this part.
2. (b)Any person called to work on a signal system or signal-related technology on which they have not been certified shall immediately notify the railroad or their employer that they are not certified to work on the signal system or signal-related technology.
3. (c)After FRA has approved a railroad's certification program pursuant to this part, no railroad shall permit a person to work on a signal system or signal-related technology on which the person has not been certified and qualified, unless the person works under the direct and immediate supervision of a mentor or qualified instructor in accordance with § 246.124.