I'm far from an expert on railroads but I've read security analysis and Graham had some stuff in there about them.
I think I remember reading that Buffett figured BNSF would benefit because they have more rails going out west than other railroads, and he believes there will be above average growth in freight traffic there due to population growth in the west and growth in trade with China. I can't remember if it was Buffett that said this in a letter or if it was someone else in an article or blog. Seems reasonable though.
Costs per ton-mile go down as the amount of freight on each trainload goes up. So, if BNSF is already running a train from LA to new orleans today, and there is no one else with the same route, and 10 years from now there is 50% more freight that needs to be moved on the same route, you are getting 50% more revenue, before counting whatever price increases you are allowed. And maintenance on the track and fuel costs don't go up as fast as revenues do.