On the Builder, extreme cold seems to be a major factor in deciding to cancel, more so than snow.
Also, on the western LD trains, if a train gets extremely late they will cancel a trip to stop accumulated delays from continuing to snowball.
Cancelling any train is usually an independent decision for each one. There is little to no linkage.
The reasons for cancellation:
1. Weather related, such as extreme cold, extremely high winds, very heavy snow. Hurricanes.
2. Line blocked/unusable. Freight derailment, avalanche, landslide, flooding, bridge burned, etc.
3. Equipment availability/very late inbound equipment. There is a limited time available to turn consists at terminals. If a train is very late, the turn might depart late. Such as the Builder recently arriving like 16 hours after it was set to depart. They'll cancel trips to stop it from snowballing and get the equipment rotation back on schedule.
I think your chances of your train going are as good as any other midwinter day. Your chances of being on the next day's train in case of cancellation are excellent, now is not a season of high load factors. Of course, if you have a sleeper there is always a chance of losing your accommodation if they're all booked.
The reason why 7(12) is cancelled may be because 8(9) appears to be cancelled. There is no inbound consist to turn. Of course, right now the EB is screwed up enough that there may be a consist sitting in Chicago, so weather (cold) may be a factor.