It is my understanding that every axle on a modern train has brakes.
According to this page https://www.engineeringtoolbox.com/friction-coefficients-d_778.html clean/dry steel on steel has a (static) friction coefficient of .5 to .8 and a tire on a dry road has 1.0 or so.
Trains weight more, BUT the normal force for placed on the wheels should go up in direct proportion. If the (max braking) force of friction is the coefficient times the normal force, then I would expect that the max braking force would also proportionally go up with more weight.
So it seems like the only thing different between a heavily loaded truck and train affecting the (max braking) force you could generate is the coefficient of friction. Because this is about half, I would expect the braking force to be half.
Kinetic energy is 1/2 * mass * (velocity)2, which gives you units of joules. As a unit, joules can be re-written as newtons of force through meters of distance. If we have half as many newtons (because half as much coefficient of friction for steel on steel vs tire on pavement) we could assume 2 times the number of meters to stop the same mass.
On top of that, it seems that increasing the mass wouldn't increase stopping distance (assuming the brakes and discs survive the increased energy dissipated): an increase in mass should proportionally increase the kinetic energy AND the maximum static friction (= max braking force), causing the two to cancel for a same stopping distance.
So why is it "common knowledge" that trains take miles to stop but not trucks? Assuming that the braking systems on a train car had the controls a truck does to stay in static friction (anti-lock braking) why can a train car not be expected to stop in about 2x the distance of a truck (in ideal conditions, of course)? And if that can be expected, why can't we expect a whole train of train cars to stop in about 2x the distance of a truck?
(And maybe it could be even less if the train can do eddy current braking to the rails where a truck has no such option.)
Edit to add a few assumptions:
- I'm assuming from the same speed (when comparing train vs truck).
- I'm assuming that the surface that we are stopping on is built to take it: the rails won't topple, splay out, widen gauge, or otherwise detach from the surface below, asphalt/concrete doesn't ripple up, any bridges the vehicles are passing over can withstand the horizontal loads imposed, etc.