Tom Anderson wrote:

Plus,
something that hasn't been mentioned, i think, is that the Canonbury
curve is single-track - the tunnel there isn't high enough to take
two full-height freight trains, so it was converted from two tracks
to one in the middle (where the arch is highest) when it became an
important freight link.

I was told by an engineer on the ELL project that the Canonbury Curve was
single tracked at electrification because the curved sides of the tunnel
roof were too low for catenary, not too low for freight trains.