WF39 and WF37 - approaching Parsons Green eastbound.

WF39 is a conventional two aspect semi-automatic signal.
WF37 is now almost unique on London Underground. as can be seen it is similar to a shunt signal but with a letter 'C' included. This is a 'Calling On' signal and dates back to when trains were routinely coupled and uncoupled during the day and on the main running lines.