I am a "foamer" (slang for a railfan) and a commentary in the April issue of Trains magazine prompted me to make some comments about shooting people on or around railroad tracks.
Union Pacific Railroad (and this applies to other railroads as well) has issued a very firm policy about photography on their right-of-way. It can be found HERE and HERE. Of particular note is the last line in the photo/video policy and I'll come back to that later.
You might think "Well, we can hear the train coming." Guess again... Trains are not as loud as one may think and by the time they get into sight it's going to be too late. Once the engineers sees you on the tracks, he's not going to be able to stop in time...
If you're shooting on the right-of-way there are a number of things that might happen. You might get away with it, you might be told to leave, you might get a citation for trespassing, you might get arrested and all your gear confiscated or, worst case scenario, someone might get hurt or killed.
Sure, there are a lot of inactive rail lines in the country, but do you know if they are truly inactive? And even if they are, they still belong to someone, most likely one of the railroads and you're still trespassing.
And if you do get away with it, take note of that last line of UP's policy I mentioned above: "Union Pacific will seek removal from publication any photograph or video that violates this policy." This is of particular importance for those doing Seniors because I'm sure your Senior and their school would just love to have the railroad go after them to have all the offending shots removed from the entire yearbook run.
If your client absolutely has to have their picture taken on tracks, then seek out a rail museum near you and find out what it would take shoot on their property. I'm sure most museums would be glad to accommodate you in turn for a donation or the payment of a fee.
I work around trains a lot as a volunteer at a railroad museum and the best advice I can give anyone about shooting on the tracks is:
DON'T DO IT!!!