"a mower on the side of the train would cut it and burn it for fuel."

The sad problem with all 'simple ideas'.... the grass, once cut, cannot be burned immediately as it has too much water in it - it must be dried. Attempts to burn wet grass are probably well know to the pyromanics among us - works great as a smoke screen generator! and you lose so much energy from the 'wet burn' that you can probably even extinguish the fire...

If you leave the grass (in a very thin layer only!) on the side of the track to dry - if no one steals or nothing eats it - you have to keep turning it so it dries properly without moulding or composting...

Not say the the dangers to tramps - refer our extensive database of folk music :-) - walking along the tracks...