In Europe, the passenger trains own the rails, they are smooth, fast and designed for people.
In the US, Amtrak rents tracks from the freight companies. They are NOT smooth, go much slower (lots of rocking, especially in the back of the train) and frequently "pull over" so a 150 car coal train can pass you.
I have traveled both quite a bit, once with a kid and more without. My learnings:
- Trains can be delayed by several hours to days. If it is crucial you be somewhere, add in a couple of extra days. My trip from Sacramento to Chicago got snowed in near Reno, the train could not make it down to Sac. We were able to hop the Coast Starlight to Portland and then caught the Empire Builder across instead of the CA Zypher. If we had not looked online and acted FAST, we would have missed our entire vacation to MI.
- PACK FOOD!!! Our kitchen car became disabled and was left behind somewhere in Montana, and we did not get one for another state or two. They brought on KFC, DH is a vegetarian. We had Clif bars and snacks for him.
- If you have a LO, put them in a back pack or carrier. DD was 6 months old and LOVED walking the length of the train. The Dome Car and the library car had the most interest for her.
- Since we were traveling several days, we got a sleeper car - LOVE LOVE LOVE those! Private room for nursing/changing, private potty, tiny shower, beds for napping/sleeping. Ours had a table, so DH and I played travel games.
- Old people love kids, commuters do not.

DD has traveled from SF to Portland with her grandparents and was the belle of the ball and spent time on the Dome Car. On commuter trains we sometimes take, people are working or reading and seem to prefer it quieter. Pack games and toys, maybe a DVD player or laptop with DVD player. But kids are kids and need to run off energy, most people get it.
-Be flexible and go with it. We love to travel by train, and cannot wait for the highspeed from SF to LA. We can mainline to DLR!