In message m, Martin
Underwood writes

I hadn't realised that the extension south to Staples Corner was built as
late as 1977. Whereabouts was the bit further south that was never built?

It would have continued to parallel the railway for another couple of
miles south (roughly to West Hampstead) where it would have joined the
infamous North Cross Route:

http://www.btinternet.com/~roads/lon...ans/ncross.jpg
--
Paul Terry