It is a retention tank for fluids draining out of the engineroom sump. Some locomotives have an extra tank bolted to the rear of the fuel tank for that purpose. Years ago when the engine room sump got full, a pipe on the right or left side sill would start to drool the overflow out on the ground, now it goes into the retention tank. Unfortunatly, vandals have discovered the retention tanks and have been caught opening the valves to drain the oily contents onto the ground (one that was arrested thought he was stealing diesel).