Now when i was driving trucks and forklifts, when we had to load something say, way up in the front of the trailer (especially shorties [pups]) that was heavy, we used a jack/stand under the front of the trailer (ahead of the dollies). ofcourse you have to add the 5000lbs of the forklift your moving that with, too. thats alot of weight ahead of the front dollies. trailers do flip! thus, jacks/stands. remove when the back half the trailer is finished loading, and its ready for a tractor.
now ive never loaded/unloaded railcars. however my time riding in the cabs of SR 38's was switching a huge paper mill/plant. they shipped out huge rolls of paper in boxcars. i saw them drive right in with a roll and back out again. repeat. and i dont recall seeing the car tip. though i was inside, same level as the forklift, which is floor height of boxcar. so i couldnt see its underframe or trucks nor could i see if it was jacked.
do railroads or shippers jack cars for loading or unloading? seems logical you could jack a car if it had a 'lift here' plate on it.