I don't recall it being an FRA requirement. However, most railroad's safety rules usually require cars to be separated at least one car length before going in between them to adjust couplers. In the last few years, they have added the formal "Red Zone", "3 step" and/or "set and centered" formal notification process to be sure the engineer does not move the train.
The railroad industry seems to be having problems with training these days, and many of the "obvious" safety proceedures have been formalized. i.e. The FRA now requires that the conductor record on a form every time they through a mainline switch in dark territory that is not subject to restricted speed. The engineer must then initial each line. Seems the railroads was not bothering to get it through eveyone's head what happens when a train hits an open mainline switch at speed. Gotta love those bureaucrats - safety through paperwork!
I have no intention of walking back looking for the conductor to find them "coupled".