In message ,
Tom Anderson writes

We had a very brief argument about this a month or so ago; Barry Salter
said they did, i said they didn't, and Clive threatened to excavate as
much of north London as was necessary to find out.

This is not an authoritative answer, but i did finally notice that
according to Quail, they do cross, and the crossing is indeed a little way
south of Harringay Green Lanes; probably too far to build a station with
interchange, though.

On a more general note, how can one go about getting accurate information
about the routes of underground railways? Are there some sort of official
maps somewhere?

There is the map at the end of ...

http://www.londontransport.co.uk/tfl.../marketone.pdf

.... it is not wonderfully detailed, but it does show that crossing point
south of Green Lanes.

--
Paul Terry