Interesting that there’s so many air-rail passengers catching the CZ at DEN. I considered doing that, but found that taking the train all the way from ARB (and connecting in CHI) with a roomette to GSC wasn’t much more time/money than flying to DEN and taking the train since I’d have to get a hotel room overnight for the 8am departure (and to be safe on the return). And I have more flexibility to change in case UP decides to do track work on the Moffett Tunnel route and does a bustitution/Wyoming detour. Of course if you’re coming from overseas that isn’t an option.
A little-known fact is that Denver is in the same category as Seattle and Chicago for the percentage of air-rail passengers on Amtrak. This is in spite of passengers having to figure things out for themselves. Often I have met Europeans who flew into Denver and then caught up with the CZ westbound.