Points Backdrive - what is it?

[ledwardz]

Back-drives assist the point machine to physical drive/shove the metalwork and keep the relevant gaps between the stock and switch rails.  Point machines drive the tips of the switch rails, back-drives are further down the switch rails.  Some organisations and switches may have more than one back-drive.
A, B, C etc., is a PWay term to describe switch types where A is the shortest and each type gets longer (in UK parlance).  So the term is literally a back drive for a D type switch (points in signalling speak).
Hope that makes sense.

Perfect! thank you very much for the response.

Lee.