The explosive often wasn't enough to damage a full steam engine. What it could do was terrorise the crew and guards. The ones I heard about didn't have a detonator. The guards had a habit of burning dead rats to get rid of them. Often in the fireplace of the guard shack. The goal was to throw fire and hot metal into the guards gathered around to stay warm.

The other advantage of a rat is that most guards wouldn't look too closely at it. Someone with lumps of coal might have them looked at, if they are caught. They are stealing coal, after all. Someone hunting rats, for food, the other hand? Poor and disgusting, get out of here and take the damn rats! You have an excuse to be there (other than sabotage) and nothing they will want back.