Well, though the bankers are usually twin locos or triplets, only one loco called the 'master' controls all the locos. Basically, control signals for all the other locomotives in the bankers set are sent from the lead locomotive. So, if the controls or the brakes fail in the lead locomotive, the whole set becomes uncontrollable. The main loco doesn't really do any work when operating with bankers. The main braking effort still comes from the bankers.
The problem here is that the train runs on a 1:37 gradient, which is the steepest gradient on BG in IR. And on top of that, there are many sharp curves, In fact, except for the crossing stations, there is literally no...
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section where the entire rake is on a straight section.
So, if a downhill train has a brake failure, note that the train will keep on accelerating because it's going downhill. And because of the curves, it can derail even before it reaches any siding. And the derailment will cause the wagons to spread all across the tracks. And this section runs on the edge of the valley for more than 30 km. Not only will the derailed wagons spread across the tracks, many of them may even plunge into the valley.
Other than a catch siding, there is no way of doing a controlled derailment of a runaway train, except in very rare cases where the terrain is suitable ( which is definitely not possible on this route).
Even if the LP of the main loco applies the locomotive brake to slow the train, again on a curve, the coaches may derail due to the sudden application of the brakes ( we see trains jumping the tracks due to emergency braking even on plain routes), so it is not a risk worth taking. Even the slightest miscalculation will lead to nothing less than a disaster.