NBC's Today did a report on photography on tracks a few years ago, and re-ran it after this last occurrence.
Rossen Reports update: See how long it can take to hear a train coming - TODAY.com
In this video, it's hard to make out the train until it's pretty close, and the train shown was going only 25 mph. I imagine in less urban areas, the train is probably going at least 55 mph on the stretch of track in question.
Granted, this doesn't really answer the question of a photographer facing the train and not seeing it, but in any case there might not be as much time as you think to get out of the way.