I am on the train, useless, but without me the train does not work, what am I?
The passengers or merchandise
They are useless to the operation of the train, but if there were no passengers (or merchandise), the train wouldn't need to work.
I have a bit of a different answer...
I know, not what you expected. Here's why it makes sense:
On a train of a dress, without a hem, the train is going to disintegrate as it's pulled across the ground, as fraying edges get worse. But the hem doesn't really do anything, it just exists.
I'll take a stab at it.
A long shot, but
does seem to fit
Could it be
Generally useless but needed for certain engines.