Ah, the wild west. At the time The Runaway Train was published, Doctor Who had only attempted an on-screen western-style story once, way back in 1966. Considering how ridiculously bad "The Gunfighters" was, I was interested to see how this tale would turn out.
But then I remembered something; at the end of the day, every AudioGO exclusive is the same. The Doctor and his companion stumble into the setting, assemble a motley crew and defeat the aliens; this story was no different, it just happened to have the American Civil War as a backdrop. I wouldn't go so far as to say it was waste of time, but it was short and forgettable with a painfully predictable plot that, to be quite frank, bored me. At times I thought I could see what the author was trying to do, but he just didn't quite make it; for example (mild spoilers), I think Oli Smith deliberately portrayed the aliens in stereotypical flying saucers in the desert because he wanted to imply that it was that incident that created the flying saucer cliche in the first place. However, these ideas just weren't articulated well at all. The only saving grace was Matt Smith's reading; his American accents were grating at first, but after you got used to them you couldn't help but smile every time he rattled off one of the Doctor's lines.
In summary, if you have an affinity for westerns or are desperate to hear Matt Smith's voice in a fresh story, give this one a go. Otherwise, think twice about wasting your time and money on something that's mediocre at best - consider a New Series Adventures Novel or Big Finish audio story instead.