Controlling the electricity flow of a model railway

I want to create a model railway with switches that control the electricity flow on the lines. I want to make these switches have serial ports so that I can connect them to my computer to control the flow.

I have the programming side sorted out, now I have to do the switches. Trouble is, I have no idea where to start. I'd love some sort of quick explanation or a link to where I can learn more.

Thanks.

You get Electronic switches which will turn on and off based on the voltage you input.. Like say 10 volts and 5 volts where 10 is ON and 5 id off..

how does that work? does it have a resistor that takes the voltage down 5 volts or something?