You would think if the overcrowding was that serious along that route they would have run more than 1tpd. My guess is it was stock that needed to go to Willesden tmd for stock swap around so they just decided to run it as a passenger service to willesden
It definitely was because of overcrowding, and it only ran once per day as it used a unit that was meant to be the spare at Willesden Depot, so by pressing it into service there would be no contingency should one of the other units fail, hence the very limited peak hour diagram.
The reason for running it in passenger service was to provide a service to Hampstead Heath for Royal Free workers, without the rather inconvenient interchange at Gospel Oak.
Not quite.
Basically it was introduced by TFL as a so called "PIXC-buster" (Passengers In eXcess of Capacity) to deal with hideous overcrowding in the peaks on the GOBLIN Line since passenger numbers had soared since TFL took over the service and the new 2 car cl172 units were quickly overwhelmed and leaving passengers behind in the peaks.
It was introduced in September 2011 as an unadvertised extra service running from Woodgrange Park at 07.59 to Upper Holloway; capacity and pathing constraints meant that it could not run in service from Barking, and it could not run into Gospel Oak as the bay pl3 was occupied at the time it would need to terminate there.
Then in December 2011 after campaigning by the GOBLIN rail users group it was extended to terminate at Hampstead Heath in order to provide interchange with the North London Line, as an alternative to terminating at Gospel Oak which was not possible. It ran as 2J96 at 07.59 Woodgrange Park to Hampstead Heath (SX) before continuing ECS to Willesden Depot.
From 2013 2J96 was extended to run in service all the way from Woodgrange Park (dep 07.59) to Willesden Junction Low Level, as the prior arrangement of terminating at Hampstead Heath and locking the unit out of service there before running empty to the depot, was taking too long and leading to "over-dwells" at Hampstead Heath delaying other westbound services on the North London Line. Extending 2J96 in service to Willesden Junction also gave the benefit of relieving some overcrowding on the NLL, and reducing the need to change for many passengers.
Interestingly in order to fit around bakerloo line services at Willesden Junction Low Level, 2J96 was pathed with an extended dwell (8-9 minutes) on the connecting spur from the NLL to the Low Level platforms, which meant that for Willesden Junction it was faster to change at Kensal Rise onto the following Richmond service which would arrive at Willesden Junction before 2J96.
The service came to an end in 2016/2017 in conjunction with electrification works and the cl.172s being transferred to the West Midlands. Presumably with the huge increase in capacity provided by the 4 car cl.710s there is no need for any additional PIXC-busters and hence the service was not reintroduced following the GOBLINs electrification.