Well, what can you say? This is a bit of a mixed bag...
In all fairness, there hasn't been a train simulator to date that can be called definitive, in my opinion, but Train Simulator is as close as it gets for rail sim fans.
First, I must say that what it offers is what you expect, but fails largely as a whole outside of the driving experience. By in large this means the graphics department. I know there are simulators out there with good graphics (Euro Truck 2), but this sadly is not one of them.
For the title of the game to tout "2013" it sure doesn't live up to 2013 standards. The game engine not only looks bad and performs bad, but dampers the whole experience. To describe it best: it seems like the old MS Train Simulator engine updated just a bit. So graphically speaking it's at the 2002-3 levels? However, the train models don't look half bad.
Outside of this the game engine can be downright clunky as there are numerous bugs and glitches that can really hurt immersive gameplay - not to mention ruin a whole mission.
Furthermore there is a massive divide between urban and rural areas. The urban areas look much better. It really appears like the game was geared for urban environments so I highly recommend that if you purchase DLC, get an urban environment.
Outside of gameplay there are issues as well: the infamous DLC. Naturally, the player would expect these to be reasonably priced, but soon will find out that you often have to purchase an engine and a route as well - which can cost around $20.00 each!
But aside from the inconsistencies Train Simualtor is probably still the best choice. It's a nice little time waster and can be quite amusing at times. If you can look past the awful game engine you can find something to enjoy.