O'Hare International truck. Airplane access for firemen and EMTs. It's the 1960 "Stairway Unit" of the Chicago Fire Dept

http://www.overdrive.fi/forum/threads/vanhoja-valokuvia-ameriikan-raitilta.267070/page-225#post-3053698

and here is it's 1980 replacement, thanks Steve!
http://www.usfirepolice.net/il_illinois/il_chicago_retired_airport_ohare_stair_634_(3).jpg