The interstitial freqs like the 453.8375 freq are newer licenses. They appear to service local facilities at discrete locations with smaller coverage areas / lower power transmitters. I'd also suggest 472.9375, where you'll find the Chicago Police mass transit unit - primarily the marked units which follow major bus routes. This is also used to contact certain suburban PD's with CTA facilities, such as Oak Park. There's some other security contractors out there, too. Might try 464.800 (100.0) and GMRS repeaters. Buses transmit GPS info on the mobile side of the 453 Mhz (458 Mhz) repeaters.

Regional Transportation Authority
*WQRV822 - 20-5wt portables on 456.3125, 456.7125, 457.1125, 457.7625, 467.4875 - for Office Building Communications