"Funnel" is a holdover from the early American wood-burners which were the same sort of moveable fire hazards as locomotives. Think of those sky-scraping early Yank riverboat chimneys &c.

The proper name was a bonnet chimney. It's a second, outer tube wrapped around the exhaust to serve as a spark arrestor. The funnel shape collected the hot cinders at the bottom. The name stuck around after the switch to coal/oil &c.

The grey/gray funnel line as slang was around before Tawney but I suspect it didn't get really popular until WWII.