LMS Jubilee 4-6-0 No 5563 Australia double-heading with a Royal Scot 4-6-0 on an up express in Tring cutting in c1937.
Ian R Townend says "I think this was taken in the summer of 1937 as 5563 has the block style insignia which it received in Feb 1937. The train reporting no. W100 is probably a North Wales Euston express and 5563 was shedded at Holyhead in 1937"

Photo: G.R. Grigs / Rail Archive Stephenson