Pennsylvania Railroad- Long Island Rail Road CT-290 Special Instructions for Employees in Electrified Territory May 1, 1933
21. Emergency jumpers are kept at places designated in the timetable, to supply power to electric equipment for short movements, where contact with the third rail is lost. Employee using jumper must guard against injury or burning by arc when contact is made or broken. Prompt report must be made to the Superintendent when (a) emergency jumpers are not at designated places, (b) difficulty is experienced in their use, (c) defective, and (d) removed from and not returned to designated place.
The current version of this document has interestingly enough the definition of an emergency jumper, but no further mention or instruction on it.
As an aside, I think in the TA these were called BULLWHIPs and you would see boxes marked the same along the track in interlockings.