Thieves typically aren't the smartest people around. Clearly there are better places to get 'free steel' that don't require as much labor or processing before you can get money from it.
But, if someone that needed steel for the steel and not to resell it they'd likely have all the required tools and vehicles for making use of it. They just need to figure out the fastest way to collect it. Rail is a dense load of steel. So if they can drive alongside a section of track and pick up several thousand pounds of steel in a couple hours I bet it would pay off.
On a related note, the office building I was working in had a two foot section of copper pipe stolen off of the air conditioning unit. Probably sells for $10 - $20? I honestly don't know what that's worth. But to get it fixed costs about $400. That's the stuff that's really annoying. I can't imagine the repair costs for the track and locomotive. Easily into the thousands, if not tens of thousands. Almost makes me wonder if the A/C repair guy went around breaking a bunch of units to drum up business.
Rather than stealing metals they should open a "low cost demolition" service. Hire out for destruction and removal of stuff. "Yeah, we'll haul it off for you. And get paid for it!"