I have made a breakthrough (for me at least) in computer control of the railway.  I have an arduino which controls power to the railway, making a locomotive go or stop.

This is achieved using a rotary encoder (a knob, in short) and a MOSFET.  I’ve not got it to reverse direction yet, when I do I’ll post pictures and code.  I’ll be doing that using a H-Bridge chip if I have one, or a DPDT relay.  For power I’m using an old PC power supply unit, which helpfully has 5v and 12v supplies – 5v for the Arduino, 12v for the locos.

I bought a couple of mosfets, relays and some other bits from bitsbox.co.uk who sell components in low quantities at reasonable prices.