Photo taken by David Backlin in June 2011
BH Photo #202762
You are right--the bridge still carries a short length of track that seems to see use as a siding connected to the Missouri Pacific (now UP) line running N-S through town. The missing bridge in the middle was the Kansas City, Clinton and Springfield Railway, later part of the Frisco.
I don't believe this bridge to be completely abandoned. I live in Garden City and go to Harrisonville all the time. Most recently I recall seeing rail cars stored on this bridge over the summer this year, so I would think it still sees storage duty at least.
I believe this is the abandoned Burlington Northern (as of 1981) line which continued north along US 71 through Belton and into south KC. North of Grandview, MO the RoW has been covered and a couple of other bridges removed. I suspect this one bridge remains only because there wasn't steel worth removing. The bridge over this line on MO 7 east of Harrisonville was removed and the cut filled several years ago. This line continued east to East Lynne where the 1954 quad shows it as part of the Frisco system.