Keep in mind also that UP tends to run dedicated sets during rush on the same route every day. On UP, there are four of these on the west line, and at least three on northwest, plus one seven car set on the north line; in fact, cab 8422 (and the other eight trailers) have been assigned to UP/W trains 32 & 55 for the last several months. Other trains, such as UP/NW runs 653 & 662, and UP/W runs 13, 36, 47, 62, 71, & 18 run with two dedicated nine-car sets that rotate on these runs every other day. Note that a car or two in any of these static rush hour consists may get swapped out periodically, but the rest of the equipment plus the cab car tend to remain intact, and generally assigned to the same route otherwise. Of course, these sets will eventually rotate off, but it can take months, though it commonly happens within days after a "modified" schedule was run. Additionally, most of the six-car 700 series rush hour trains on ME also run with dedicated consists, and I suspect that once Metra ever gets their hands on more money for newer cars, I'd guess that more of the older equipment will see rush hour service only before being retired...
--Dorian--