I second the suggestion to not look out at things close to the train, but look farther away. I've only traveled on one Superliner (double decker train), and found the sway on the top level to be noticeably more than on the single level trains, but not enough to bother us. A lower level seat or compartment might be a better choice if sway bothers you.
Off the top of my head, I'd say a train has more in common with a boat on a lake than with a deep sea fishing boat, although a train is usually smoother than a lake boat at speed. There is obviously motion, but not the deep rolling motion you might get in rougher seas. Of course, it's hard to say how it would affect any particular person. In my experience, it doesn't seem to be a very common problem, and you don't find barf bags in coach as you do on planes, FWIW.
Maybe that's encouraging.
You can buy "sea band" bracelet-type thingies in drug stores, and two of the teachers I worked with swore by them when they were pregnant and having problems with nausea. (Teachers don't have frequent access to restrooms.) The bands put pressure on a point on your wrist and they both said they helped them a lot. They are marketed as a motion sickness preventative so you might want to take those, just in case.
I hope you have a pleasant trip.