Some numbers to help this exercise -

The weight of a modern car is max ~2e5 lbs.
The maximum tension on the couplers is about 2.5e4 lbs
maximum train length is about 100 cars, so that's about 80 lbs/car -
HUH? - the secret is that there's about 6" of slack in each coupling, so
the cars are started one at a time, and all the car needs is to renew the
energy lost to friction. For a slow moving train, that's about 80 lbs.
Engineers take courses on managing drawbar tension. And in mountain
country they use 'DPU' - distributed power units - remote control locos
halfway back.

The maximum allowed buffing force is somewhat less - the train
won't act as a column very well.

guys - you'll never get enough energy out of rockets to have any
impact on a train. GO LOOK AT A TRAIN. You'll notice they're big
hefty objects.

as for the "air rocket" idea - forget it. You're many orders of magnitude
off.

Some engines used to have steam brakes. AFAIK cars never did. Steam
condenses, it'd
be hard to run it down a trainline.

If you've got air on the car, the best use for it in an emergency is into
the brake cylinder.