Engine Usage on Erie/EL in Binghamton

I've noticed in many photos of the Erie and EL in Binghamton that they often placed an engine or two where the red arrow is. It was almost always larger road power as opposed to smaller engines (Alco switchers or GP9s).

What was the purpose? Thanks.

Seems it was normally power assigned to PB-100 and its' counterpart the Piggy Back train from EL to the D&H. D&H brought the train into Binghamton and cut off and EL power was there waiting for it to move it west / or vice versa.