[Scuderia_Turini]

Picking up from where Andy left off...

There are four diodes for each lane. These provide "full-wave" rectification to channel the AC into DC. That is that two diodes pass the current when the AC is one way around and the other two pass when the AC is the other way around (Alternating Current). 

The other two diodes pass current from each power supply circuit to the LED (feeding via the single dropper resistor). That way the LED lights up if any one of the poweer sockets is being fed and also if both are.

The green components are simply capacitors to provide a little smoothing of the DC current going to the lanes/cars, acting as suppressors. 

The switch connects/disconnects the two power lines. This is after the rectification stages so that the power-packs are effectively isolated from each other by the diodes. 

The introduction of these power-bases and the accompanying three-wire controllers was a step up for Hornby/Scalextric but it has all been undone in the more recent power-bases and controllers. 

Perhaps Hornby were inundated with service calls concerning operator error with the switch and therefore they decided to dumb-down back to the simple arrangement.

The carbon-fibre look was very trendy! Below is how mine looks nowadays.

Leo

PS - A Nerdy bit - these power-bases are constructed using "discrete components" where each item has its own single function. Pretty old-fashioned technology in the age of "chips" which combine several functions into one component.