I'd recommend TRAINZ Railroad Simulator 2006.
With TRS2006 you can create whole worlds with finely textured scenery, hundreds of structures, many types of track (standard gauge, 2ft & 3ft narrow gauge), hundred of pieces of rolling stock. Included are also complete layouts with 'scenarios', where you have to do a job as engineer.
From the internet you can download literally thousands of layouts, rolling stock, structures, bridges, tunnels, landscape textures and... and... and... I think there is much more second party stuff (most of it free) around than for the MS Train Simulator.
One word of caution: Perhaps you might find the older version TRS2004 for a bargain price. Don't buy it! Running it is a pain in the ***!

:curse: Loading times are extremely slow and most of the time it is running very jerkily. The 2006 version has significant software improvements, resulting in higher frame rates. Also the new download manager works much better than the older one.
Ok, TRS2006 isn't exactly cheap (about $40), and it is a 'hardware hog' - but if you want to run trains (either in easy 'DCC mode' or realistic mode) and want to create your dream landscape with trains, this could be what you are looking for.
Here is the
homepage of the TRAINZ makers. Hope this helps a bit!
Ron