Well, if that happen then the Loco gets changed after completing its course like:
1. LKO-BSB-LKO (Different Loco to commence next course towards BSB)
2. LKO-BSB-CNB-LKO (Loco changed to move towards BSB)
The Loco may get changed for routine overhaul/check-maintenance/other train. Now next time, u both can ask the LOCO to get certain answer!!