How fast can trains go?
user
That depends on the kind of train, the length of the train, the density of the track, the grade, and the direction of the train.
assistant
What about coal trains?
user
I’m not sure what you mean by “coal trains”.
assistant
How fast can trains go?
user
That depends on the kind of train, the length of the train, the density of the track, the grade, and the direction of the train.
assistant
What about coal trains?
user
Probably they’d be slow, but faster than bicycles.  Also, their length will affect how fast they can go.  More metal = faster.  Less coal = slower.
assistant
How fast can trains go?