I have just been looking at OS maps of Camden Road, which has an interesting aside. As originally constructed it had platforms on the Midland main line slow lines (but not apparently the fasts), and until some time after 1896 these would have served both main line suburban trains out of St Pancras and trains coming up from the Metropolitan Widened Lines, which joined the main line at St Paul's Junction, south of the Camden Road tunnels. By 1916 the Met Widened Lines had been extended northwards to Dock Junction (and on to Kentish Town) via a new tunnel parallel to the Midland ones and in the process the Camden Road platforms appear to have migrated across to the new tracks, so would only have been available to trains from the Met. The station was closed in 1913 so it's hard to know how long the new arrangement was operational for.