DCC, Digital Command Control, is THE standard for digital control of model railroads. It is based on developments of the german company Lenz and was declared a standard by the NMRA and in the meantime got a lot of enhancements and extensions.

The +/-12 volt signal on the rail is a stream of short (binary 1) and long (binary 0) pulses, for example

More info you can find on wikipedia.org - Digital_Command_Control - and lots of details, open hardware designs and open source software on OpenDCC .

The author of this website is currently building a few - very simple - projects around DCC, like
a throttle with interface to the Lenz system
a very basic and simple DCC command station (not yet finished)
a "processing" program for the offline evaluation of DCC signals.
.....