With thanks to the Model Railroad & Misc. Electronics web of R Paisley.
This is a very useful site for all sorts of electronic circuits for model railways.

Using "AT" Type Computer Power Supplies.

The PSU has + and - 12 Volts and + and - 5 Volts

A view of my power supply attached to the side plate of the layout.

The small transformer on top is 16V AC for CDU etc