Isnt the line from SRR fully electrified?
For many trains passing through VSKP, one WAP 4 is detached there and it waits to get attached to some other train to bring it in or out of VSKP.
For ex:The Loco of incoming 2704 in VSKP, takes out the 12805 Janmabhoomi Express; and secondly, the Rake of 2704 undergoes direction change in VSKP. The Loco which takes out 12805 upto Tenali, also brings back 12806 upto VSKP.