For what it is worth, in Oregon at least, reporting marks are required to obtain permits for public grade crossings. Thusly Sumpter Valley Ry. is officially SUVX despite being 25 miles removed from any other railroad. These reporting marks are only used on paperwork however, and none of our equipment is so lettered. I am not familiar with all of the FRA required reports but I believe the reporting marks are used there as well. They are on the engineers and conductors reports as well as crew logs.