Yes, as GregoryGrice said it is an Automatic Speed Control test loop. On a regular basis, each lead car must have its speed control system tested. That box contains a switch which activates an oscillator to permit this.
The Automatic Speed Control System was introduced after the horrific wrecks in 1950. It includes simplified cab signals and an arrangement which applies the brakes if the train passes a more restrictive signal without the engineer taking action.
When the M-1's came in 1968, the covered signal aspects were increased from three to seven. The original M-1s were equipped with "Automatic Train Control," an ASC enhancement. ATC permitted an engineer to place the controller in an ATC position and the train would automatically increase and reduce its speed in accordance with the signals. The train would also have selectors to set the destination. Wayside "readers" would read the trains destination and switches would automatically line their routes; a system that was in fact tested in Valley Interlocking. The destination roll signs would also automatically adjust.
None of this worked very well, and it was never used with the exception of the seven aspect automatic speed control which all subsequent MUs and road engines shared.
