I spent 77 hours traveling from California to DC by train. It was worth it, but there are things everyone should know before any long ride
The observation car fills up quickly, but you can find great views just below it.

Everyone wants to be in the observation car during the most scenic stretches of the Zephyr's route — especially through the Rocky Mountains — which makes it tricky to find a seat.
If it's full, head to the café car, which is right below the observation deck on the Zephyr. There are tables right up against the large windows, and you'll have almost as good a view as you'd get upstairs.
You have to buy something to sit there, but I thought it was worth it for a change of scenery.
It can get cold on the train, so remember to bring a jacket and comfortable clothes.

Thankfully, the train cars are air-conditioned in the summer. However, that can also make them quite chilly, especially at night.
I came prepared for my rides with some thermal leggings and a puffy jacket.
Make sure to wear clothes that you're comfortable sitting in for extended periods. I also recommend bringing a change of clothes (or at least an extra shirt) if you're traveling for multiple days.
It's possible to get free hot water on the trains.

I realized on this journey that the café car had a supply of free hot water. You can bring your own food, like a cup of instant noodles, or tea, and just ask a staff member to fill up your cup.
I saw several passengers doing this and wished I'd thought to bring some just-add-water meals.
There were also spigots dispensing drinking water throughout the train, though they were pretty slow to use. I wanted to avoid buying bottled water, so I made do.
Prepare yourself for possible delays, especially on trains traveling longer distances.

The railroads on which Amtrak trains travel are owned by private companies that also cater to freight trains. That leads to a lot of delays, as Amtrak waits to let other trains pass.
A longer route like the California Zephyr's can fall behind schedule by hours each time.
I was lucky that my train to Chicago was only about an hour delayed, so I ended up having two hours to make my connection to the Cardinal. However, these potential delays are something to consider when purchasing a ticket with a tight layover.