When I was doing a bridge inspection over a CSX line, they wanted notification if we were doing anything within 25' of the line as they wanted a flagger. Then the flagger showed up and said actually, it's 7'. (The nearest bridge pier was a squeak over 7' from the track and not in very good shape, but then it was over 100 years old...)
I'd imagine that at 13' from the rails you're pretty darn close to, if not inside, the ROW, so just call them up and stumble through their bureaucracy and find out what they require... and remember that trees get bigger and have branches. The FRA may have requirements, but the railroads are all privately owned, so what's okay for CSX may not be okay for someone else.