Is there a Railmaster size . . .

. . . between the 42mm 2502.52.00 and the 49mm XXL 2806.52.37?

I am looking for a larger watch, but the smaller 42mm version is hardly bigger than my Seamaster 50th Anniversary GMT, and the XXL is probably too large for my wrist.

Thank you.

No I don't think so.

I have the 42mm Railmaster.
From the dimensions it may not seem bigger than your Seamaster, but - on the wrist it looks bigger than it is because of the dial design and a relatively thin bezel.
It's a lovely watch - one of the best looking that Omega have made for years (IMO) - but I hardly wear it as it looks too big on me, and I don't have particularly wimpy wrists..

From what you say, it could be fine for you, although finding one to try may not be easy.

See pic for comparison - both watches are 42mm. The one on the left I find an OK size to wear. Apologies about the poor picture - took it very quickly just to show.

(You're right about the XXL - it looks the size of an alarm clock, and of course has a different movement).