Wait until the head end passes, then run for it. As the car approaches, jog alongside. Throw your pack on first. Put your hands on the ladder or on the boxcar door as you run. When you have a real good grip, pull yourself up.
Nothing feels quite the same.
Getting off a moving train is the same thing in reverse. Wait until the train is going slower than a jog. Don't make the mistake of my friend Faceplant Dave. Throw your pack off first. Hold on to the car tightly and start running as your feet touch the ground.