Going back to the original question, I've heard talk that basically one unit is all that can be spared. Any others will come from the removal of units in daily service as maintenance cannot be skipped.
However comments over the 378 use on GOBLIN may be more to do with actually getting the units fully cleared. Too many assume that just because a train is cleared by NR to run through a station, it is ok for passengers. Its not. There is a long process where stepping distances, lighting levels (both day and night time) and something that each class of train is unique.
One example of this is that during testing of the class 700 it was found that one of the new build NR stations that the new energy efficient lighting wasn't actually up to the standards required for when a train arrived but had been fine under 377/387 equipment.
The 378s will be held to the latest standards on the GOBLIN as its a new routes and even the existing infrastructure might no pass now if they were being introduced now. The 378s use an old DOO equipment, one GTR and the unions have discussed about replacing for DOO usage.
It's a complex process and if we are to believe the statement from TfL about it, it seems more that the drivers haven't been trained on the 378s yet as a result of TfL being busy to check if they are compatible, not because they don't want the drivers input. Just don't confuse compatible being with only the infrastructure on and around the rails.
Customers on London Overground's Gospel Oak to Barking line will see a mixed fleet of trains operating from Monday 28 January, as an interim measure until the delayed new electric 'Class 710' trains are delivered by the manufacturer, Bombardier Transportation, and are ready for passenger service.
The Mayor has also spoken directly with Bombardier and secured a month's free travel for passengers once the new trains are fully introduced. The details of this are being developed and will be communicated closer to the time.
Given earlier comments about the lengthy process which would need to be followed to use 378s in passenger service the latest TFL announcement seems quite emphatic. Does one suspect that the process has been condensed to the absolute minimum, and presumably they have secured OK from the unions to go down this route.
As for the announcement itself it echoes my earlier view in the main Goblin thread that TFL PR people will have a hard job selling this as an "improvement" (more potential capacity) when it really means one in two Goblin services have to be axed.
It will also be interesting to see quite how the free travel offer is implemented especially for people who use Goblin as part of a longer trip using other TFL services or Oyster. Quite a lot of people will presumably still be charged for further travel in zones 2, 3 or 4 if they go beyond the Goblin route - in which case they may save nothing at all?
According to postings elsewhere the current diagram requirements are 26 for ELL, 25 NLL/WLL/DC lines. This is 51 diagrams for 57 units. Taking the three for GOBLIN leaves three units not in service on a daily basis. For a fleet this size that will be likely to be two exam units and another for mods/faults.
This basically means one fault and you end up with trains cancelled all day.
And and remember the pool of 3x 4 car 378s will require 100% availability from the end of February until the 710 gets 4 or more units into traffic.
EDIT: Just check and so far there is 8x 710s in London. Enough to run the entire GOBLIN service.