Illinois Route 129 is a short fragment of a route in southwestern Will County. I honestly couldn't tell you a reason why it's still signed. It runs from an incomplete interchange with Interstate 55 near Wilmington, south to Illinois Route 113 in Braidwood, across a set of railroad tracks from Illinois Route 53. It can't really be used to get anywhere far away, since it's less than four miles long and its northern end can only access Interstate 55 to or from the northbound lanes.
The northern end of Illinois 129 is essentially a "right-in, right-out" junction with Interstate 55's northbound lanes. IDOT plans to expand this interchange to provide more access sometime in 2023 or 2024. (A tip from a local: to access a full interchange with Interstate 55, coming from Illinois 129, continue northerly on the freeway's eastern frontage road to the Lorenzo Road interchange.)
The southern end of Illinois 129 is an interesting situation, where Illinois 113 crosses a set of railroad tracks but intersects two roads that lie immediately on either side of said tracks. The road on the east side of the tracks is Illinois 53, and the road on the west side of the tracks carries Illinois 129 north of the crossing. In the mid-2010's, IDOT built permanent signals at this location. Eastbound on Illinois 113 gets its own phase, as does Illinois 113 westbound. Illinois 53 traffic on the east side of the tracks has its own phase, plus a left-turn phase, and the road on the west side of the tracks, home to Illinois 129 on the north approach, has a separate thru phase and a left turn phase. This creates one signal that has six phases, meaning that, in the interest of keeping everyone safe, vehicles often spend a lot of time waiting at this oddball junction.