I don't think it would take much to get the rail car rolling based on my memory watching a string of rail cars roll off into the distance when I released the brakes on them when I was a kid visiting my grandparents. I lived in that town as an adult and many times I went by that section of track I still marveled that there was any slope at all.
You couldn't see it with the naked eye.
The pick up could do it. You might have to put some weight in the bed, but hey, I do that to my personal pick up truck just to make it safe in the winter. Put enough weight on the drivers and it will pull it.
However, if you are going to do this with regularity, be careful how you spec out your pick up truck. Good low speed rear end(s) and extra heavy duty auto trans (this would be heck on clutches) with a REAL good external cooler. Most of what you will be doing will be with the transmission in a high slip stage.
Be sure to PM the transmission regularly too. That kind of service will cook the trans fluid rapidly.
I once pulled a large bus-less weight; about 15 tons several miles across town to a shop with a pick up truck. The terrain was relatively flat but pulling the bridge over the river was heck. The truck did a good job. We rarely got over 20-30 mph. (The bus would run so as to air up the brakes but had a drive line problem so that it wouldn't pull itself.) I always wondered how much I aged the tranny in my truck that evening.
rmw