That definitely is an odd place for a Receptacle!
Not 100% sure, but I may have the answer.
If this Track is located where Passenger (Commuter) Trains are "made-up", or stored temporarily - at the last Station of the Subdivision, then the Outlet may be for Temporary Power to the Passenger Cars while the Head-End Power from the Locomotive is not available.
The Cars would be powered from the Outlet, while the Locomotive is either:
* Uncoupled and away for Servicing / Refueling,
* Main & / or Auxiliary Prime Movers are at idle,
* Locomotive is shut down.
The power is made available for Coach Maintenance (cleaning, etc.).
Normally, the AC Power would be supplied from the "HEP" (Head-End Power) package in the Locomotive, via the Auxiliary Generator connected to either the Main Prime Mover, or the Auxiliary (HEP) Prime Mover.
If the Outlet has a Receptacle, rated for 480Y/277V 3Ø 4 Wire + GND., either in the 20 Amp, 30 Amp or 50 Amp range, the chances of this Outlet being used for Coach Maintenance is close to 100% (>99.523008863%)
Only one point of connection would be needed, as the remaining Cars will be connected via M.U. Cables.
A "Patch Cord" with (most likely) a Locking Plug on one end, and a Pin/Sleeve Cap on the other would be used to connect the Cars to the "Temporary AC Power".
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!