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In depth idiot
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... I have seen a lot on underside of layouts, but found no real explanation of what is being used or for what.
The copious wiring on many layout undersides was a direct result of two factors on DC layouts, with 'cab control' connected to multiple switched sections to enable independent operation of locos, and parking locos in isolated sections; and operation of point motors with track supply switched on the live crossing. This was the typical standard to which club exhibition layouts and ambitious private layouts were constructed before DCC conquered the world.

The two factors leading to the copious wiring on such DC layouts.
Firstly, the necessary switched power supply to every layout section and point motor.
Secondly, the inability of many to implement common return wiring, to the extent that some declare it impossible.

A relatively modest forty sections and sixty point motors DC layout, requires 40+60 supply wires and 1 common return, or 80+120 wires if common return is not utilised; running from the source of power and control around the layout.

On a DCC layout four wires, two for track power, two for point motor busses. (These can be wired common return for just three wires, but in practise the physical routing of the two busses is often more conveniently arranged well separated.) There are then many small 'droppers' connecting the track power bus to the rails, and 'fans' of local wiring from accessory decoders to point motors.
 
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