Question

When starting a heavy train, why will train engineers sometimes back up, stop, and then proceed forward? (This technique is called "bunching slack.")

Answer 1

it is kind of in the name. it brings the cars closer together then continues to go forward.

Answer 2

maybe....
when going in reverse it makes a tiny gap between each boxcar, then when going forward it pushes or pulls each car one at a time to build momentum rather than moving them all at once...not sure