I don't think this one has been seen yet, I built it back in 2005 when I couldn't access the Forum.
The car is a free-lance weed sprayer, based on one of my Hudson pattern skip frames (see this thread for chassis details http://forum.gn15.info/viewtopic.php?t=8967
The sprayer itself is, IIRC:
Two air tanks from the old MDC 'Hustler' in G scale -- a resin block from a commerical project (bolster snubber from a GE "Genesis" loco in 1:32) -- a headlight from the same 'Hustler' kit -- a Bandai 1:48 wooden box from an old German 'Maultier' kit -- piping from a Bachmann 1:20 Shay cab -- lever from an Atlas O scale switchtower -- and the sprayer nozzle head is a handrail piece with the ball ends. The paint is kinda lumpy, I didn't have an airbrush compressor at the time, but heavy weathering will hide those sins.
The idea is it would take a steam connection from the locomotive via hose, to operate the pump itself.
Just noticed this frame uses the laser cut pedestals I experimented with.