==== TL;DR version ====
I'd recommend this game with caveats IF you can get it cheaply during a sale or in a bundle. I would feel a bit cheated by the lack of quality control if I'd paid full price.
==== Full review ====
First, let's note the genre we're talking about here. This is a simulator, not a game as such; it's a bit like Flight Simulator in that if things are exciting, you're probably not doing very well. While it does have some less realistic modes, people who want a "toy trains" sort of game should probably look elsewhere. However, if you want a taste of what it's like to take a mile-long freight train over Tehachapi Pass, using fully realistic cab controls, this is aimed solidly at you.
The locomotives are carefully and lovingly detailed, both inside and out, and each have their own unique sounds and handling characteristics. While I'm by no means a rivet counter, I found the physics of both the overall train and the locomotive itself to be well-modeled, or at very least plausible. This goes for both the diesel-electrics and the steam engines, which are a physics challenge all on their own.
The graphics feel dated; they're about what you'd expect of a big-budget game circa 2005. They're certainly sufficient for the purpose, but I wouldn't play this game just to sightsee. There's a nice mix of terrain and parts of the world, though, including the US, Britain, China, and Russia. In spite of the relatively dated graphics, the game requires a fair bit of oomph to run. Those whose gaming rigs are more than a few years old will find this game to be jittery. There are also object and texture pop-up issues that can be distracting given the continual forward movement.
Both the routes and driving sessions are essentially fan-created. This is understandable considering the effort involved in modeling hundreds of miles of railway. Unfortunately, though, this means the quality varies wildly. Some sessions are particularly well done, especially on the ECML Kings Cross-Newcastle and Mojave Division routes. Others are frustratingly buggy, sometimes to the point of being impossible to complete.
I did not try any of the multiplayer features, so I can't comment on those.
Overall this is a reasonably engaging simulator, and the non-buggy sessions provide a nice spread of challenges. If you're the type who needs to complete EVERY achievment, though, the buggy ones will frustrate you.