There is only so many of them they can use. What you may have to do, unless some Rail fan who just happens to know what they are on stops by, is search...or program..every Rail frequency and then lockout the ones you know are used by BNSF or UPR and then just listen. One thing about their spectrum is, if you are using a scanner with selectable frequency steps, (5/10/12.5/15khz etc.) you can put in the low end and the high end in a Search and set your scanner to 15 khz steps and basically scan each individual channel. Here is a link, if you don't have one to all possible channels for Rail. Railroad Radio Frequencies
I used to live in Eastern Washington and see the Palouse RIver Coulee City railroad running west of Spokane and kind of wonder what frequencies they were on but never pursued trying to find them.