In message . com,
Neillw001 writes

You don't get 450's running down to Epsom as far as I know.

No, but couldn't they have been held up by reversals at Wimbledon?

There just weren't any trains running through Motspur Park, hence
spare capacity on the mainline until Raynes Park.

But how were the slow and fast lines being used, given the very unusual
service pattern involved?

I think the unit at Earlsfield had
failed. Why would you stop a train with just two carriages in the
platform and detrain all the passengers otherwise?

Possibly so. Possibly the driver had been advised that the queue on the
down fast was so long that it would be better to detrain passengers from
the rear onto the waiting down slow train, rather than waiting to pull
in fully.

I have no idea, but the extensive disruption due to the Motspur Park
works seems the most likely cause - perhaps exacerbated by a breakdown,
as you suggest.

--
Paul Terry