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Detail of standard London Underground points locking mechanism.

The PL & D box is the point lock and detection box.  The box is shown here with the cover removed and it has six contacts, tree for the normal position and three for the reverse position.

The ground lock secures the points in the selected psition by means of a air operated plug.

Standard London Underground point machines are driven by compressed air as it was always considered faster than electrically driven points.  Modern electric points are now replacing some older types.

Photo by Donald McGarr.