The Rainbow Bridge in Riverton KS, (Baxter Springs is the closest town) dates back to the two lanes period, and is directly adjacent to a much newer version of 66.
One of the things you realize driving route 66 is it developed in stages. There’s the original one, which was generally two lanes and existed in a time when there wasn’t much traffic… then increased usage necessitated wider bridges, or that the road be moved entirely… and then there’s the most recent version some of which I’m beginning to think was organized AFTER the road had been decommissioned, when it started to become a tourist destination.
Apparently, in a fight between the county, which wanted funds from the government to build the new bridge whose rules demanded demolishing the old bridge, and the Kansas Route 66 Association… who clearly wanted it preserved, a compromised was reached. And now the bridge is listed with the National Register, so it will never be demolished, and has since been used as local for filming route 66 specials on TV.