Trainplayer is a program that's been around for a while, and I'm admittedly late to the party, but the good thing about coming late is that things tend to be in full swing by the time you arrive.
Unlike 3D "virtual model railroading" software such as Microsoft Train Simulator or Auran's Trainz Railroad Simulator, Trainplayer is a decidedly 2D proposition. Specifically, the program starts with a bitmap image, which can be a scan of a drawn or printed trackplan, or output from a CAD or graphics program. Overlayed invisibly on the image is vector graphics information that defines the track plan for the software. This allows you to then drive one or more trains on top of the track plan, throw the switches, couple and uncouple cars, etc. Frankly, the software is an "armchair" model railroader's wet dream.
You might think this approach to computer railroading is less sophisticated than the aforementioned 3D programs. I suppose it is. It's also a hell of a lot of fun, and lately my 3D simulator programs have been gathering dust in favor of this one.
Some neat features have been incorporated into the recent version 3.0, including the ability to run trains automatically, either by recording your moves, or by creating text scripts using a rudimentary programming language. The latter, in my opinion, expands the utility of this software greatly, allowing one to grow from the role of engineer to that of dispatcher.
By itself, Trainplayer allows you to do everything except import your own graphics and create your own track plans. For this, you'll need the more expensive Tracklayer, which incorporates Trainplayer.
The basic package (Trainplayer or Tracklayer) includes ten layouts taken from the classic book "101 Track Plans for Model Railroaders" by Linn Westcott. For more money, you can expand the number of layouts incrementally, going up to all 101 plans. There are also a wide variety of user plans that can be freely downloaded and installed from the Trainplayer Web site.
In sum, if you're particularly interested in operations, or if you're in the planning stage for a real model layout, or if you just like playing with trains, I can't recommend this program highly enough. You'll be hooked.