Steam Locomotive and "A" Tank Questions

[nw-mailing-list at nwhs.org]

Greetings,

   I have a couple of questions regarding steam locomotives and tenders. 
First, what was the general date on which coal bunker extensions started 
appearing on N&W steam? Would this have happened all at once, or generally 
as locomotives were shopped for repairs, and was there only certain 
locomotives to receive this "upgrade"?

   What was the purpose for sheathing the smoke boxes of the A's? (not sure 
if that would be the correct term to use)

   Regarding the "A" tanks, there were 2 main classes of these, correct? 
What were the differences between the classes of "A" tanks? And would a 
certain class be assigned to certain locomotives?

   Thank you!

Michael Stovall