Budd cars on the Blue line...

These are the oldest cars that CTA currently operates, and all of them are currently assigned to this route. IIRC, some of these cars from 2200 to the low 2300 series or so spent some time during the '70s and '80's on what was previously known as the Lake/Dan Ryan line (before the routes were realigned and color-coding route designations were adopted). However, there are a certain group of cars, starting at about 2321 or so, through 2352 that have spent their entire service lives on that route, going on 35 years by now.

I think this is partly due to the fact that these cars were purchased for by the City of Chicago for use on what was then the new extensions within the Dan Ryan and Kennedy expressway medians to 95th St. and Jefferson Park respectively. In conjunction with this, I remember reading somewhere that these cars weren't supposed to be used on any other line except on those that had median expressway running for a portion of the route, although I readily admit that may or may not be entirely accurate.

All of which makes me wonder: This might be a tough question to answer, but has there even been a case during CTA's (and its predecessors) history where a portion, or even the entire series of a certain type of rolling stock were assigned to only one route for their entire service lives? Anyone that knows, TIA...