I take Amtrak about every year, usually the California Zephyr.
The train will be late. Just expect that and then it's a pleasant surprise if it actually arrives on time. Two hours late is normal. After four hours it kinda loses it's place in traffic and things can get bad fast after that. My previous record was ten hours late but I beat that a year ago at 15 hours - I didn't arrive on the expected day. Don't plan much for the day you arrive and don't arrange any tight connections.
Some of the equipment is old, from the 1980s or so. Some seats have a cutout where the ash tray used to be.
Coach seats are very comfortable with much more leg room than a plane or bus. They recline pretty far and have leg rests. Seating in coach will vary from station to station. Some places assign seats, some board by the size of the party, and some are a free-for-all. There is overhead storage for luggage, a space on the lower level for larger items, and a baggage car for checked items.
Roometts are small but private and surprisingly comfortable to sleep in. Actual rooms are much more expensive but have lots of space to move around. The sleepers have an on-board shower. Sleeper passengers have access to the Metropolitan Lounge available at some stations. The one in Chicago is very nice with free snacks.
Trains have a cafe car to buy snacks and a small seating area. The dining car is usually just for sleeper passengers. Food is microwaved but actually very good quality. There's fresh coffee in the sleepers in the morning.
Some trains have an observation car with comfortable seats and large windows. It's a great place to watch the scenery and meet people. On the western trains I've seen lots of antelope, elk, bald eagles, wild horses in Nevada, an occasional bear or moose. The Zephyr goes thru the Rockies, passing thru some canyons that are otherwise inaccessible except by kayak. One section of the Colorado is known as "Moon river" because the river guides encourage their clients to drop their shorts and moon the train. The Coast Starlight and Surfliner go right along the Pacific coast in places with beautiful views of the ocean.
Car attendants are generally courteous and helpful. Some conductors will get on the intercom and point out the sights or give a history of the area. There's generally a brief stop every four to six hours where you can get off the train and walk around for a few minutes.
The nice thing about trains is that when you arrive, you're right there - you don't have to take a shuttle or rent a car to get to the city, you're already there. In Chicago you're a block from the Loop. In NYC you're already in Manhattan. For San Francisco, you're in Emeryville but there's an Amtrak bus that meets the train to connect to the city. Trains feel like a more human way to travel. Airlines treat you as a threat and an annoyance and they'd be happier if you'd just buy your ticket and stay home. Trains can be fun and relaxing, they just require patience.