Our intension on recording is bring the mic near the sound source as much as possible to lower other ambient sounds around. For wheels on track with wind etc. we have some recordings downloaded from internet but other train specific sounds we need recording ie locomotive engine,fan is different from others, and there are train specific warnings,bells etc. So for that purpose isnt shotgun mic right choice?
Again, if you're going to be close to the source, then no, not really. I'd say unless you're going to be more than 3 feet or so away, there's no reason for a shotgun mic. Any closer to that, and you're going to want the area sound - when you hear a train, you don't hear this one little piece of it, after all. It also depends on the environment you're recording in. Up until your most recent response, I didn't think you were recording a real train. Now it sounds like you are. Based on what you are trying to do, this might even be the more difficult way to go about it. But if you are recording a real train, then you most need to be careful of your environment. If you are in a noisy area, you're going to have a lot of post processing work to do on this audio, regardless of the mic you use. For most sounds (except the horn and engine sounds, off the top of my head), you'll probably be better off recording something indoors that you can manipulate to sound like train noises.
My battery rolling example is something you can pitch shift to make it sound deeper - like a heavy wheel rolling along. If you do this on a metal surface, you'd probably get an even better result. This kind of thinking is what makes nice, clean, usable sound.