If you've never listened to Railroad Radio, you've missed an opportunity to learn all about how freight moves in Colorado. It takes a bit of patience and knowledge of the nature of railroad operations, but, having it on in the background, you'd be surprised what you can pick up.
It's also a good indicator, given how expensive gas is recently, of how busy these rail routes are on a given day. Careful listening will help you know where the trains are, and how likely a photo run will produce good results.
All three live audio feeds are linked below and in the near column.