Bruce, what Tascam recorder are you using? I ask because most of them besides the basic DR-05 have multiple inputs. The DR-40 has two inputs (plus two built-in mics for 4 tracks total), and there are some that have 6 tracks with 4 inputs. I watched a video of TCS recording an Alco, and they had mic's all over that locomotive recording it from many angles. I'm wondering if at least getting the recording from two angles would allow you to get the best of both the prime mover and the exhaust. Recorded as separate tracks, then combine them on top of each other and edit to play in Railpro.
I have a nice Sennheisser mic I can send your way if you need a second one.