Apologies if I got the interim rolling stock wrong

If as a temporary measure slow line Euston Watford Junction had adapted ex Subsurface (ie main line gague and carriage size) trains, there would be three options:

A stock: On Met and formerly East London line, problem with that is that its body is multi piece and riveted which could react with arcing overhead current on its roof if not regulary checked there.

C Stock: In four car format could work but multi door (four sets, largest number for metros in the UK at present) so could be cold on the outer London bits beyond Hatch End / Harrow and Wealdstone

D stock: Most like Class 378s so could be cascaded to Southern region if adapted for 3 rail dc current or kept on Hatch End- South Hampstead / Euston services. Trouble is that its current shoes that pick up under floor rail line power have historically arced and this could react with overhead power lines.

Hence though it would be difficult to budget for, more dual voltage 378s in four car format would be the most realistic option from operational considerations if the Dc lines were ever to be overhead electrified in part.

However Croxley Rail Link requires 4 rail ac tracks from Bushey Station Dc lines to Watford Junction for the new Metropolitan Line S Stock.

As such since Bombardier also built that further S stock could be built in dual voltage 4 Rail and overhead format (with option for 3 rail running as well if possible). That way Some 4 car S stock format could run from Watford Junction to Harrow and Wealdstone or Wembley Central to connect with the Bakerloo and if there is additional match day demand during any future World Cup bid.