First things first, is what would have stopped its closure?
Lets say, its because the Northern Heights project went ahead after all, and the Northern Line continued to use the Northern City Line to Moorgate. This would mean, that by the time the GN Electrification scheme would come around, you would have three choices of how you could wire the GN Electrics to Moorgate:
1. Rebore and wire the Widened Lines tunnels from Kings Cross York Road-which would be difficult to do.
2. Forget wiring to Moorgate. Keep GN trains at Kings Cross and use Finsbury Park as cross platform interchange with the Northern Line to Moorgate-not a pratical plan because Kings Cross would get very busy in peak hours.
3. Keep GN trains going to Broad Street.
I think number 3 is what would happen. All you would have to do is electrify the Canonbury Cruve and the 313s could run down the third rail to Broad Street.
Inevitably, this would breathe some life into the station-however North London Line services and Watford DC services would still be run down and ultimately diverted from Broad Street in the alte 80s. Broad Street would be given the same treatment as Charing Cross, and turned into a huge 1980s style office development oppressing the station platfroms below.
By the late 80s, Broad Street would be closed on the weekends. Network South East would come under pressure to close the station and redevelop the site. The 1989 Central London Route Study would be very interesting aand I wouldn't be surprised if it involved a plan to close Broad Street and replace it with a Crossrail type route to somewhere south of the river. This would still never be built as the Chelsea to Hackney/East-West Crossrail combination would still be the best value for money.
The ELL would still happen I think, because the Broad Street viaduct was built with four tracks in mind. So the ELL would continue past the BR tracks. So the ELX would still proposed in 1990s as it was in our world.
Through the 90s, not much would change, however I wouldn't be surprised if Thameslink 2000 included the GN electric lines in its plan so the Broad Street viaduct can be used for the ELL.
Even if this is the case, there would be no political changes so Thameslink 2000 would still happen 20 years after it was meant to open.
By the early 2000s, expect the SRA to put forward proposals to use the Bishopsgate Goods Yard as as bridge into Broad Street to take Shenfield Metro services away from Liverpool Street. This would not be possible by ELL extension works of course and faced with a choice between the ELX or helping Shenfield Metro passengers politically I think the ELX will pervail.
Broad Street would still be open by 2019 and would be open 7 days a week. It will grow in importance when Crossrail opens and Canary Wharf will be only a few stops away. I would expect the Canonbury to Broad Street section to be quite busy especially with people coming from Dalston into Central London as the East End becomes more developed. We would look back as that London Station that for many years would teeter on the edge of oblivion but would one day come into its own, and we'd be very glad to of kept it.
