Each train does not have it's own set of equipment. The equipment gets used on multiple runs on the line. This means the equipment (and crews) need to be turned around (cleaned, stocked, crew rested and so on) before the next run of the train. IIRC there are actually 3 sets of equipment that run this route. The equipment used on 351, also gets turned around and runs on 354 from Chicago to Pontiac. 354 is scheduled to arrive at 1:18 am, meaning the crews that clean, stock, and do everything else only have about 4 hours and 25 minutes to do everything that is required. On the schedule everything works out well and they should be able to do that.
Now, what happens when 354 gets delayed somewhere along the route. NS, CN, or another railroad they need to deal with has their own problem. There are issues with passengers on a train, or getting on or off a train.
It's also not just freight railroad issues they may have to deal with. 354 will at some point need to meet 355 along the way. What if 355 is delayed, so there is a delay in where they can do the meet. We aren't talking about double tracked mainline all the way from Detroit to Pontiac. I've witnessed numerous times where Amtrak has a train waiting in the siding in Chelsea because they have not arrived in Chelsea at the same time. Some of that may be built into the schedule, but often not enough so the trains run late.