Living in North London (a mile or so from Dalston Junction) I used the North London line to and from Broad Street many times. That stretch is mainly on viaducts and overbridges (once you get beyond Richmond Road). The major civil structure was a huge girder bridge which crosses Kingsland Road at a skewed angle before the line goes on to cross the eastern end of Old Street. This stretch is now used by the East London Line extension from Shoreditch to Dalston Junction. Broad Street was largely deserted during the day and in a shocking state of disrepair. In the peak hours some services served the Great Northern lines north of Finsbury Park, usually hauled by Brush D55xx (latterly class 31) locos. They gained access to the North London line by way of the junction between Canonbury and Highbury & Islington with the line routing under Highbury Fields, joining the ECML between Drayton Park and the Emirates Stadium.
One of my early jobs was in Aldwych, adjacent to the Law Courts. I'd travel to Holborn by Piccadilly Line from Kings Cross (73 bus to there). I knew the times of the Aldwych shuttle fairly well and would get one if it was convenient but if not it was quicker to walk down Kingsway. Trouble was the cross platform interchange for the Aldwych branch was from the northbound Piccadilly Line platform, so it was up and over to change from the westbound platform (but my legs were a lot younger then!). The lift at Aldwych had a ticket office in one corner. Before leaving Holborn the guard of the shuttle would press a large brass plunger which would ring a bell in the lift at Aldwych. The station bod (who seemed to do everything) would then take the lift down to meet the train.
I travelled the Ongar branch a number of times, usually courtesy of a "Twin Rover" (red buses and the Underground, but not north of Rickmansworth). Blake Hall was something else. We alighted there once to have a mooch round before catching the returning train to Epping. There was simply nothing at all apart from a few isolated houses and farm buildings within walking distance of the station and it was a good ten minute slog up to the A414 Epping-Ongar Road. I only did that once! I'm not surprised it was the least used station on the system (that dubious distinction now being held by Roding Valley). There were reports that it handled fewer than 20 passengers daily before its closure which was on 31/10/81. On a visit to the E&OR last year it seemed little had changed. The platform is still extant though the station building has been converted to a private dwelling. I took some photos of it during its conversion (the builders very kindly let me have a sniff around) but I cannot lay my hands on them at present. The rest of the Ongar branch closed on 30/9/94.
I managed to get to Croxley Green just once, but more recently. I did that in the early 90s in anticipation of the services being ceased. They were "temporarily" suspended in 1996. I hope nobody missed the last train! I had previously managed Watford Junction on one of the few Bakerloo peak hour services and the sheds where those trains were stabled overnight were still evident in the 1990s.
A trip to the Widened Lines from Finsbury Park, stopping at Kings Cross York Road was an occasional adventure. Only did the return once. It involved travelling through a long curving, steeply inclined tunnel up to a platform (similarly inclined) on the suburban side of Kings Cross. The early Craven DMUs sometimes had a struggle.
I used Holborn Viaduct for a few years. I worked for a few weeks in the office block overlooking the platforms (Williams National House, demolished when the line was re-routed through the Snow Hill Tunnel and City Thameslink station was constructed).
I managed Palace Gates just the once. I travelled from Seven Sisters and I think it was about a year before the services ceased.
Finally, in September 1961 I had a ticket for the last train from Baker Street to Amersham, where the changeover from electric loco to steam was made at Rickmansworth for the last time. Electrification was extended from Rikky to Amersham only and the LT Aylesbury service was cut back to Amersham the day before. I cannot remember which of the Met's electric locos made the final journey. Oh for a digital camera then! I have in my mind that the Chesham shuttle lingered under steam for an extra day or two and I seem to remember a steam loco being in the Chesham branch platform as we passed Chalfont (but details are a little hazy!).
I read somewhere quite recently that at one time LU ran a post theatre train from Aldwych beyond Holborn terminating somewhere further up the line.
Yeah, it went to Finsbury Park, back in the days when that was the Picc's northern terminus. Pretty sure they stopped running pre-World War 2 (I don't think they survived WW1 by very long).
My information is that it didn't even make it to WW1 and that it only ran from 1907 to about 1910.