What is special about the northbound ELL in that area that makes it "common sense" to leave early there?
We have 16 trains per hour from Surrey Quays to Dalston. When you have trains coming from 4 different places, 3 of which occur delays from Southern (honest but true), it's causes a bottleneck at Silwood (outside Surrey Quays). Heading northbound from Surrey Quays, there are only 2 destinations, Highbury & Islington or Dalston Junction, so it's more of a turn up and go service.
The signal sections are incredibly short on the ELL "core", it creates a lot more pressure on the drivers running on restrictives for the entire journey so to ease that and the congestion it was decided to abandon the timetable between Surrey Quays to Dalston Junction.
The works at London Bridge have caused considerable knock-on delays to our services and it's now very rare we see a green in the 'core'.
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In terms of the last train, especially with my post above, we stick to the time. The last train of the evening will run no matter what! Last month I was the last northbound train from Clapham Junction to Highbury, there was a points failure between Wandsworth Road and Clapham High Street, we were delayed by about 50 minutes but I had to sit it out and wait to run in service.
I can't speak for the GOB or North London Line, as they have a different Control to East London Line, but on our side for the last train I often see station staff on platform to see off the last train. At Highbury the platform staff there will close the gate at the top of the stairs and give us a wave that the last pasenger is coming down.