Paul Corfield wrote:

Well I've spent a few minutes with Google Maps and I'll agree with Mr
Thant that there is more space than I imagined there was.

Back in the pre-Google Maps era I explored the area IRL and came to the
conclusion that the only "structure" (which does not include the flat
surface of Jarrow Road) preventing 4 tracking from Coppermill Junction to
Broxbourne junction is the Eleanor Cross Road bridge. While it isn't
entirely surprising that the new bridge carrying Leeside Road over the
railway was built *long* enough to allow four tracks, amazingly the new
bridge to carry the railway over Mollison Avenue was built *wide* enough for
four tracks - I can't see how the COBA worked out for that one, with no
prospect of quadrupling in the air at the time.