Most probably, the LGD WAP7 loco stalled on the gradient because of continuous wheel slipping. Dakshin Exp happens to be a 22 coacher, so yeah! :P
Wheel slip generally occurs when the tractive effort applied by the loco exceeds the adhesive limits of the loco. In wet weather, tracks become slippery.
A simple example :
Case 1 : Running on a dry floor....
more...

Case 2 : Running on a wet floor.
In the second case, you're likely to slip and fall, right? Why so? Friction reduces because of moisture.
Likewise, in case of locos which haul trains with the help of rolling friction (plus a little bit of sliding friction), wheels keep rotating in their place with hardly any movement of the train whenever wheel slip occurs. That's why locos have sandboxes (or sanders) to improve friction on slippery rails.
Video : Wheel slipping of a WDM3A loco.