I have spent many hours happily playing this game, but I can't really recommend it as a "game" because of a couple things:
1) Framerate, audio, entire application responsiveness takes a massive hit without severely limiting texture qualities and draw distance, making for a high cost to actually take advantage of the game's optimized graphics options. Two upgraded computers later, I still have to cripple the draw distance and texture quality in order to play without the game regularly freezing up for a few seconds every time a bunch of new content needs to be drawn, such as when you pan the camera around.
2) Included sessions (for which there are steam achievements) are often glitchy or tempermental in their willingness to aknowledge your progress. A signal which should change automatically doesn't, resulting in you being unable to complete a session after investing an hour or more into it. Or a session will suggest you are driving a long train, however you are only driving a locomotive with no actual stock attached. Usually I like retail "games" to have more polish than this, and feel that this superb simulator is, unfortunately, not so superb a "game".
Further dissuading me from pushing this game on my friends is the fact that the developer, n3v/auran, has now embarked on a kickstarter campaign to fund creation of the next revision of the Trainz series, "Trainz: A New Era." I don't expect the glitches or engine to get any extra polish/performance at this point.
That all said, however, I have enjoyed this game quite a bit since I got it. The driving experience lets you choose often (in some sessions you are forced to use one or the other) whether to use DCC or CAB mode. DCC seems to emulate an electric model railroad with a simple knob to control movement. CAB mode attempts to emulate real railroads (I am not qualified to say how close it comes) by making the user interact with the reverser (forward/reverse/neutral), a range of throttle notches from -9 to 9, and, for when the engine is pulling cars and the e-brake won't cut it, a complex train brake where you cycle through initial -> lap -> application and watch the various brake pipe/cyl pressures adjust as they eventually bring the train to a stop. The graphics can be nice, and seems many in-game objects have detailed dynamic behavior, from trees which have different graphics for each season, to train cars which have specific loading/unloading animations. The content update hub is also a place for users to post their own content, and for other players to add it to their own collection, not unlike the steam workshop. One can download locomotives, stock, scenery, routes, sessions, etc. which the community has uploaded simply by searching the interface and choosing to download it. The system addresses revisions by content publishers in the community so that new revisions are included when other players with their content perform normal content updates.
The game has a surveyor mode, where you can create your own train routes and sessions, laying out terrain, tracks, signals, scenery, triggers, session start/end points, etc.. I played with it a little, covering the wireframe ground with snow and a loop of track that had a switch leading off to a roundhouse, in which I had some community-provided engines. I got distracted long before I created anything fancy. As others have documented, building a world is a very time consuming process, and some people take months or years on works which they still consider incomplete. While working on your route/session in surveyor mode, you can fire up driver mode to drive through your creation and test it out and have some fun. If you are happy with the results, you can then publish them to the hub of community content.
All in all, I think this game is a great sandbox, with some crazy demanding requirements for full graphics performance, and a handful of glitchy/buggy sessions. While the community may continue to contribute content to the game, allowing content to continue to evolve, the engine is dated and not looking like it will ever have its performance-based frustrations addressed. For the retail price, there are almost certainly more actively developed train simulator solutions. If you already own it though, or it is on sale, you may get many hours of fun out of it.