A station at Northumberland Park

There was a serious look by LT as it was the original staff platform area which was considered with a small booking office the main problem was having no codes so that the trains speed would be very slow once the codes ceased.

Mill Hill East?

Can you argue that Olympia is a successful one-station branch? Likewise Shoreditch [was]? Both had twin tracks leading to or coming from a single line and single platform. Also, New Cross and New Cross Gate were single platforms served by single lines off a twin-track branch.

I seem to recall that when Northumberland Park was conceived that a link into British Rail was discussed in order to deliver the new stock directly. I don't know if my memory is playing tricks or not though! I cannot recall whether there was any proposal to have a joint BR/LU station.

There is also the fact that trains would have served Northumberland Park irregardless of the status of a passenger station at the site, as the trains that do run there serve as staff trains for the depot. The other single-platform branches mentioned above don't (or didn't) get a train service unless needed.

I guess New Cross and New Cross Gate count as successful one station branches. And Heathrow T5?

I drive past this depot regularly now. It's the oddest thing to see trains that spend their entire service life below ground up on the surface.

I would suggest that this proves my point.

If through trains are run from Camden Town to MHE, the Barnet line gets an irregular frequency. But a FinCen-MHE shuttle would hardly be an efficient use of resources.

It's probably also relevant that the South Acton, Aldwych, and Ongar appendages have all been lopped off.

I don't personally think of Heathrow T5 as a branch, just the end of the line. If anything, Terminal 4 works more like a branch through my eyes even though it is a loop with no termination.