How old are sliding doors?
I've googled on warehouse doors and regency warehouse doors and sliding doors (and got a lot of movie hits on that one), and historic warehouse doors... Historic warehouse got me some interesting stuff, but nothing about doors that I could find.
EDIT: Yes, I hit Wikipedia's entry on "door." No, it didn't seem to help unless I skimmed over some really relevant sentence, hidden somewhere. O:(
EDIT2: Looks like sliding tracks, on barns at least, show up around 1850. Now to research that lock-link.