Trainz 2006 is the best in model train games.
First of all, when you dive right in the game, you realise that there are 2 modes of play: Driver and Surveyor.
In Driver, the basics are very simple, you pick a route, then a session and then start to play. Some sessions require to drive the whole map, stopping at every station, in others you have to shunt freight and passenger cars into their proper sidings or to specific trains and in some, you can simply explore.
Surveyor mode is like an track editor mode. Here you can modify the default routes, by adding water, changing elevation, adding buildings, industries, trees etc. You can also create your own routes or see other peoples work, which is very cool. I would prefer this over what is in Train Simulator because is more simple, user friendly and very, very fun.
Also there is a Railyard, where you can see all the trains and cars that are in the game: Big Boy, DD40AX, TGV, Intercity HST, SD40-2, A4 Malard etc.
A problem that I saw was with the crashes: sometimes, trains won't derail at high speed collisions and will go through scenery and other things.
Trainz Railroad Simulator 2006: an enjoyable game that will be played by every gamer.