I claim no originality for this idea - I got it from a book by Cyril Freezer, and I'm sure it is widely done - but I liked the idea of the main components of the electrics being in a separate box. This keeps all mains wiring in a box on the floor, and also most attractively, allows the expensive bits to be used for many layouts! I made this one with my Dad years ago, when my trainsets evolved into model railways, so I reckon it has powered 8 layouts now ...
Anyway the basic principle is to provide the layout with all the power sources it needs, in this case a controlled 12V DC for the track, a pulsed 20V DC for points (i.e. a CDU), and a 16V AC supply for anything else - though so far I haven't found anything that needs it! My powerpack contains a transformer with 2 x 16 V AC outputs, both protected with thermal circuit breakers (this is very neccessary to avoid blowing the transformer in the event of a short-circuit), a Capacitor Discharge Unit for points (can't recall the make), and a high-frequency track cleaner (Gaugemaster). The latter can be switched out, in case of delicate motors! The schematic below should make all this clear.
All this is contained in a sturdy wooden box, the mains cable is clamped, fused at plug, with a neon to show the power is on, and of course the transformer is earthed. Some vent holes, a handle, and a couple of hooks to provide somewhere to wind up the cable complete it. There are two DIN sockets, a 5-way is the standard for hand-held controllers (16 V AC in, controlled 12 V DC out), and a 6-way for the lead to the layout. Each layout then just needs a socket for power in. See photo below.
In fact I have a second powerpack, an ultra-small one for very small layouts (one of my layouts is not much bigger than this powerpack!) containing just a transformer and track-cleaner. Points need to be manual, or as on Southon Yard, I built a simple CDU into the control panel for the single point with a rectifier and capacitor! If you wanted to avoid doing mains wiring, you could mount a cased transformer onto a board or in a box containing the low-voltage wiring.
The photo also shows the hand-held controller, in this case an AMR. They are long since out of business I believe, but I picked up a second one second-hand recently. I also have a Gaugemaster "HH" which is very similar, but I prefer the toggle direction switch of the AMR to the slide switch of the HH - it is kinder to the thumb! I also think the AMR gives better control, but I have no way of measuring it! Both are of the "feedback" type which I find gives good low-speed control. I don't run loco's far or for long on my layouts, so I have never seen any overheating issues that some find with feedback controllers and HF track cleaners.