For more than 50 years the miniature lever power frame provided the means of controlling power signalling until relay interlockings with control panels gained supremacy in the 1950s. The Westinghouse Style L frame represented a milestone in power frame development by virtue of its substitution of electric locks for the interlocking between levers rather than mechanical means which had previously been employed.

This book describes the apparatus in considerable detail, from its inception in 1928 through to the present day.  First published in 1989, this latest edition has been comprehensively updated whilst also benefiting from significantly more photographs.

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