In answer to your question about what to choose, I do not think being near a railroad should be any cause for concern at all. For it to be a real concern, I think your pool would have to be so close to the tracks that you wouldn't want to swim in it. :lol:
If you want an opinion, I would suggest going Cement/Gunite/Shotcrete if you want the best possible pool. Long term, I think they are the top of the line and best possible option, but that is my opinion and preference. Many have wonderful pools that are made from fiberglass or vinyl lined, but I prefer the above if it's in the budget. If you can go this route, I would say choose this option every time.