I am dredging up this topic once more because I photographed some signs today along the Union Pacific line as it runs through Watauga (near Fort Worth). Every so often someone either posts or asks questions involving photographs taken on the railroad tracks. There are relatively safe spots to do such photography (like an unused spur, for example). I remember in one thread that someone noted they'd never seen a sign that prohibited being on the tracks.
The signs on this stretch of the railroad, just west of U.S. 377, appear to only be in Watauga (not in far north Fort Worth north of here, or south in Haltom City). I have not contacted anyone from Union Pacific, but an incident in October 2007 may have led to the postings. Read about that
here. Watauga enacted a "quiet zone" for trains passing through before the fatality, which means engineers only sound the horn if there's an emergency.
There are procedures for securing temporary use of Union Pacific tracks outlined
here. These may apply to photographic uses as well.
Of course, if I'd been more patient, a train would've gone by while I was standing there. Was already a mile closer to home when I passed one heading north.