I agree--these look like modern beams. IIRC KCS had this line open until the '60s or early '70s serving a couple of small customers. Unless the original bridge was in terrible shape or flood damaged I can't picture why they would feel a need to replace it.
It would help if we could get some KCS "Air Line" history from a railfan.
Based on the design of the built-up beams, the existing bridge appears to be mid-20th Century.
This bridge was probably built between Jan 1891 and Mar 1892 when the Air Line opened. The Air Line was built by the man who later founded the Kansas City Southern RR, Arthur Stillwell. http://www.arthurstilwell.com/timeline.html
Looking at the Independence quad topo map, this might be the crossing for the "Air Line" that ran from downtown Kansas City, near 2nd and Wyandotte, out to Fairmount Park in Independence. Just NW of the bridge there is a label of "Air Line Junction". Few traces of the line are still there but the grade can be seen running along the dam of the lake NW of the intersection of Truman Road and Sterling. The line was a light duty car line that ended a few blocks east of Fairmount Park at what is now a city maintenance facility at Truman Road and Forest. The Kansas City Southern owned the RoW as of the mid '60s when the current topo was drawn up. Info on Fairmount Park and the Air Line here: http://www.oldfairmountpark.com/1893.html
If this is the old Air Line bridge, a lot of people rode across it 100 years ago.