Railroad carriage question

[pengolodh_sc at yahoo.no]

I am trying to find some information on British standard passenger-
carriages, particularly the older types, from before the HST-125-era, 
such as BR Mk 1 or GWR clerestory-type carriages.  I tried searching 
on the web myself, but found little, apart from sites which chose to 
display text in the form of .igf-files.  Even Hornby was of little 
help to me, so if any of the Brits on the list know fo knowledgeable 
sites (or know of some books - the library might be able to help me 
then), I would be much obliged.  The reason I am asking, has to do 
with a pet theory I have about the Hogwarts Express, but I need to 
control it before I release it on an unsuspecting audience over on 
the HPfGU-list.