I see your point that Chicago's station is a much different environment than the New Orleans station. However, my point here is that the same principles and routines used in New Orleans could be applied in Chicago. Should it really matter how many people and trains are near the nearby platforms? If an officer/conductor is guarding the train, it really shouldn't matter how much activity there is outside of the train, should there? If this really is an issue, can't the car doors/gate doors be locked until boarding? What was the procedure when the Sunset still ran to Florida?
Remember that one platform serves two tracks, so it's not that easy to guard the train as other trains can be busy loading and unloading on the other track. And a conductor certainly won't be doing it, don't want to waste his hours of service standing around. As for locking the doors, first I'm not sure if Superliner door can be "locked". They can be dogged shut, but I don't know if there is an actual lock. Second, you've got cleaning people coming and going so you really can't lock the doors, since they won't be able to get in or out. Not to mention that if passengers are on the train, you can't have a locked door for safety reasons.
You also have to pay a crew to stay on board to deal with the passengers who don't detrain and you've now go to supply hotel power to the train. Finally you've got to deal with the fact that the engines need servicing. Many times the trains pull straight into CUS, so the only way to free up/change out the engines for servicing is a run to the yard. Yes, you can back the train in, but now you're tying up more of the main at the throat of the station to do that, and then you've now got to run the fuel truck over to the station, unless you unhook the engines and send them to the yard.
Your point about how only about 1/5 of the passengers going through Chicago is interesting. Where did you get those numbers from Alan? Perhaps if these trains were marketed as a through-train, there would be more through-passengers.
More of a guess on my part than anything, and I suspect that I erred on the high side. As Wayman pointed out in his post, passengers arriving into Chicago have many choices. The odds of a high level of connecting pax to the SWC aren't good. And consider that if your idea did improve the run through business to the SWC, the near sold out conditions on the Capitol caused by those connecting pax would now hurt connecting ridership numbers on the other trains out of Chicago.
As for the train taking up track space at the station, remember that time could be saved by keeping luggage and people on the train, as things don't need to be transferred from one train to another, keeping trains on the platform.
That probably would save all of 15 to 20 minutes, if that. That's nothing against a train taking up a track for 5 hours.
This run-through of the Chief and the Capitol was put in place for a few years in the late 90s, where people and their luggage could stay on the train. I'm sure that the amount of trains and people are comparable to what they are today.
No, ridership on Amtrak has been climbing pretty steadily for the last 10 years or so, so it's higher today than it was back then. And ridership on the METRA commuter system is way up by comparison to the 90's.