Alemite, or "Block" grease from greasing the rods! mmmmmmmmm good!
Also, the smell of the steam. The oil and boiler treatment added to the water makes for a smell that becomes wonderful once you've worked with steam, and nostalgic when you've been away from it.
Same with coal smoke and Alemite grease, of course.
All these things mingle together to make a wonderful aroma that sends the wife to another room.
Mark D. - The bottom of the information curve