Create some lightweight handcar-style contraptions and stash them in the baggage car. Once you've completed the robbery run to the back of the train. Put the handcar on the tracks, step onto it, then let go of the train and apply the brakes.
Your contraption can actually be considerably simpler than this as you just need somewhere for everyone to stand/sit and a brake.
To make deploying easier I'd actually have some guides built to pull the wheels into the right place as it drops onto the track. Tie the two guides to the back of the train, drop them down and slot them onto the rail, then run the cart down the guides onto the rails, again tying the cart to the back of the train.
Once the cart is in place one conspirator stays with it while the rest do the robbery and then run back, jump on, and away you go.