Chicago is obviously one tangled maze of track - and I know in recent years the RR’s, even competitors, have really worked with each other to keep things moving. And for many point A to point B trains within the Chicago Terminal Area there is more than one route they can take. So what selects which route do they take? Do trains try to stay on company owned tracks, or are they simply routed over whosever tracks are the emptiest at the time?
Yes they stay on company tracks. For intense if you have a derailment on the up northwest line they wouldn’t send a train on that line to aurora. I have been in Chicago for a longtime
So much depends on exactly where the derailment is, and how bad it is (meaning, how long it will take to clean up). Most railroads have options over other tracks of their own if a derailment occurs in Chicago and environs. Or, they can use terminal lines (IHB, BRC) if that will help.
So are the belt lines like “common ground” where anyone can go?
Not quite. BRC is owned in equal shares by the six major Class I’s and IHB is jointly owned by CSX, NS & CP (not sure on how it’s divided). The owning roads all have trackage rights over the belt lines in which they have an interest but non-owners (WSOR, South Shore, etc) need to negotiate the necessary trackage rights arrangements for transfers, through trains, etc.