Help - Been asked a question

There are no passenger services - the last to use it were those from the Great Northern suburban stations to Broad Street, which ceased when the Northern City was handed over and BR services ran to Moorgate via Old Street.

I don't think the line was electrified until some time later though, perhaps 1987-ish? To get simple clearance for the wires through Canonbury Tunnel the line was singled, meaning a bit more work if a frequent passenger service were to be reinstated. There are no serious plans for this at the moment, although there might be a case for extending the East London line Overground services to Finsbury Park and perhaps towards Highgate. Would need some major civil engineering though to avoid conflicts.

I don't know of an online diagram, but I can describe it (and you see it fairly clearly on Google Maps).

Basically, at Drayton Park you have three tracks - the two from the Northern City and the single track from Canonbury. A little north of the station the Canonbury line splits into two, and the northbound immediately goes over both Northern City tracks, so going under the stadium bridge you have (west to east) Northbound Canonbury, Northbound NC, Southbound NC, Southbound Canonbury.

The two northbound lines then dive under the ECML together, coming out between the Down Carriage and Down Slow and connecting to each. Meanwhile the two southbound tracks also continue north, going underneath the Up Goods track of the ECML and coming out between that line and the Up Slow, and connecting to each.

For reference, the ECML layout west to east is Down Carriage, Down Slow (1 or 2), Down Fast, Up Fast, Up Slow and Up Goods - where Up is southbound (towards King's Cross).

If you take a cross section just north of the diveunders, you get:
DC---NC,NNC---DS,DF,UF,US---SNC,SC---UG.

(I'm an Northern City commuter, so I'm intimately familiar with these junctions)

There's a diagram here