Railway 32 is a screensaver program for Microsoft Windows (32-bit Windows, as the name implies - it's known to work on systems from Windows 95 on up, including Windows 2000 and Windows XP at this writing). It's the product of an Englishman, Mark Goodspeed, and it's charityware; the (small) shareware fee is donated to a charity of the author's choosing.
Railway 32 depicts a drawn railway scene in side view. Trains enter from one direction or the other, do some things 'on set' and eventually leave. What distinguishes Railway 32 from other screensaver programs like this is that it doesn't just simulate one track; instead, there is a z dimension. Multiple tracks can be superimposed, including double track and four track main lines. The track diagram can get even more complex; crossovers, sidings, stations where the trains stop, a locomotive depot, a marshalling yard, a container terminal, and more. Train routing and collision avoidance is implemented, with full signalling. The drawn trains have full transparency, so they can realistically pass in front of each other.
The supplied trains and rolling stock are all of British prototypes and quite simply drawn, but the true greatness of the program is that it is user extensible. New locomotives, stock, trains and scenery can be drawn and combined together as completely user-defined scenes. A reasonably large user community has grown up around the program and much work has been done.
Most of what is available, reflecting the national origin of the program, is for British railway scenes, but there is a fair amount of work available for American railways, as well as modules for Chinese and German railways.
The defined scale for drawing standard gauge equipment is 3 pixels per foot; as a fortunate coincidence, this is amazingly close to that chosen independently by European train screensaver programs such as Meyer and Traffic, which use a scale of 10cm/pixel, which is only about 2% different; not enough to notice. Thus, equipment drawn for one program can be readily used in the others.
Narrow gauge equipment is generally drawn to a scale of 5 pixels per foot, which does not correspond to any other program's scale (they generally use 10cm/pixel for everything).
The program can either be run as a screensaver, or in a normal window.
The main site for the program is at http://www.railway32.net/, while the mailing list is at http://groups.yahoo.com/group/railway32/.
Disclosure: I am a contributor to the user-contributed work on Railway 32.