Apart from the opportunity for Clara Clayton to have an exciting traversal of the distance between the back of the fuel tender and the engine (Golly!) in Back to the Future III: Was there any reason to leave the tender attached to the engine?
- Fully loaded, it would probably weigh half as much as the engine itself. The less mass a surface vehicle is hauling, the quicker it can accelerate.
- For fuel they were mostly relying on the color coded burn sticks the Doc used in his furnace. Even if they'd needed extra fuel beyond that, they could've stocked the furnace & detached the tender when they stopped for the car. It was not far from there to the point of '88 or bust'.
I don't think I've ever seen a steam engine traveling with no tender. So it might be that a steam train and its tender are effectively inseparable, but I am no train buff.
One might also ponder why the Doc didn't just stuff a long tube full with gunpowder, put a shaped nozzle at one end & call it a solid rocket motor - but perhaps that is for another question.. (starts hummin' to the tune of Rocket 88..)