Ratings on cars had to do mostly with bearing capacity of wheels and damage to rail infrastructure. I'm not suggesting you test it, but don't undersell the structural strength of what you have based on the rail capacity rating. You're probably correct, the earth at each end would limit how much it could support.The car was originally rated to hold 135,000 pounds, but even if is was undamaged, it couldn't hold that now, because it is being supported only by the earth it sits on.