I'd recommend the BC75 if all your are going to use it for is listening to rail as most rail is done with analog signal. Analog scanners are much cheaper than the newer digital scanners ($100ish to $500ish). As you go up in analog scanner number (the 125 or 345) there are more little features that are nice if you're using this heavily (more channels, etc.) but the BC75 should work for hobby listening.
If you want to listen to more than rail (police, fire, etc.) it depends on where you live. If you send me your zip code I can recommend a scanner to listen to all traffic in your area.
Here is a list of recommended rail scanners from Zip Scanners.