Intriguing- wonder if they're testing down to Moorgate? Or was it a 172?
Big obvious colourscheme difference between Underground and Overground- the cab side and ends are red on the Underground, but yellow on the Overground. Very, very obvious!
There are some London Overground 313s at Hornsey, maybe the OP saw one of these?
Believe me if there were 378s at Hornsey then I would certainly would have heard of them being there.
I had an e-mail at home yesterday saying a 378 was going up to Hornsey but not sure why. Tyre turning maybe?
It's 378 210.
The return to Willesden is early tomorrow ( I think 06.30 off Hornsey - excluding any last minute variation! ) and it runs via Finsbury Park and Primrose Hill, so interesting photo opportunities there for anyone who is in the area.
Is that the one that popped down to Moorgate?

Is that the one that popped down to Moorgate?