Bridge before CUT...

Okay,

For those of you who are familiar with Chicago Union Terminal. About 4-5 miles before/after (depending on direction of travel) before/after the Metra shop/yards and CUT (riding the Hiawatha or Empire Builder... not sure what direction that would be in relation to CUT), above one of the sets of approach tracks is a rusted, abandoned, black bridge (your stereotypical Warren truss-type). It's resting on some concrete abuntments, with a visible approach (buried in local flora) on one side, while the other... well, there's no apparent line there, just the factory rooflines, although I'm assuming there was a tall viaduct that connected to the bridge.

Was this part of a now abandoned railroad line, former "L" line or interurban route? Just currious.

South Side St. Charles Airline.

No, I don't believe it's a part of the Air Line, however, I did some digging around after I read your post and came across mention of the Baltimore & Ohio Chicago Terminal Railroad, could it have been a part of that network? Like I said, it looks like it hasn't been in use for many years (and I'm not 100% up to speed on all the railroads, interurban, trolley and El lines that ran through the Chi-town area). But I'll take your word that it might have been a part of the air line. (Can't seem to find much writtien on it though.)

For starters, the bridge in question is NOT part of the St. Charles Air Line---the "Air Line" is roughly parellel to 16th St and connected the CB&Q, CNW and the B&OCT to the IC.

The bridge is also NOT part of the B&OCTRR, because the furthest north the B&OCT went is parellel to the Congress Expressway.