You said that the train is moving at "constant velocity". That means that
it's tootling along on a straight track, at a constant speed.
It doesn't matter what the speed is. Just the fact that it's not changing
means that the sum of all the forces on the train ... the "net" force on it ...
is zero.
If the engines are applying 4,000 newtons of forward force to the train,
then all of the forces of friction acting on the train add up to 4,000 newtons
backwards.
