Well, I was too. Until now.
I modified the MkI a bit to come up with an LMS express passenger carriage. The graphics are slightly modded, and the stats changed to values appropriate for an LMS carriage.
The BR MkI was based on an LMS design, so it's not that out of place to use the BR MkI graphics as the basis of an LMS express carriage. Not that the Locomotion graphics for the BR MkI are that accurate anyway - it should have at least two more windows and a door in the middle...
Anyway, the LMS express carriage looks like this:
and goes like this:
Oh, and it can be downloaded from HERE. Please let me know what you think.
UPDATE: Version 1.1
Now introduced in a more prototypical 1933 - use Pikkabird's Pullman for 90mph trains in 1927. Reduced capacity to 42 (first class) to make the surburban carriage more appealing for commuter runs.