The ones I've been on has a speed indicator at the ends of each car (don't worry, it's on the inside) 

You almost don't feel the acceleration. They do pick up speed quite fast, especially when departing stations in small towns where they are allowed to go straight to top speed as there is no built up area around. Still, if you are seated you will not notice and if you are standing you will not need to fight it. 

There are sometimes a lot of sideways motion, like the track is not straight. I am guessing it is something that would not be noticeable at 150km/h, but at 300km/h you do feel it. 

I find the trains comfortable. They are wider than (at least European) trains so the compartment feels very spacious. They usually don't allow standing passengers, so you have your seat to yourself. 

If you have a first class ticket you get a small piece of cake and tea or coffee. If you are a coffee drinker, don't chose the coffee. It will make your face go sour. 

You can get barfbags on the train. I've never seen anyone need it, and it's not like the Chinese don't suffer from motion sickness, I can see that on the Mainland<>Hong Kong ferries. 

Have a nice trip. 

And a small tip. At the train stations, somewhere in the waiting area, there is a person handing out small (300ml or so) bottles of water for free if you show then your ticket. Well, at least this holds true for Guangzhou and Changsha stations. 

hmmmm,,  I have found them to be very comfy, I have rode First-Class and Second-Class, both are nice.  I wouldn't pay extra for F/C myself though, not enough difference in the quality to me.  I'm not prone to motion-sickness but I don't think they make anyone motion-sick, they are quite smooth and stable.  Seems a bit like flying where it is really hard to get a feeling of how fast you are going.  I've seen 308kph on the speedometer many times.  Bring earplugs, they are as noisy as most any Chinese gathering place.  Children run wild, which I still find amazing but which Sino's do not bat an eye at.

Ginger is good for nausea - cut up some ginger root and put in hot water. Simple and effective remedy that i find helps.

You definitely do not notice the insane rate of speed.  The trains are so smooth and comfy, you'll be surprised when the train hits the legal limit for speed, because it feels like you aren't even moving.  You only notice the speed when you pass through a tunnel, or another train zooms by in the blink of an eye.  I wouldn't worry too much.  It's much more probable to get motion sickness on a bus, ferry, or car.

Not at all. I can even look out the windows and watch.

Try this: move your feet around... up and down. Shake them up and down. It helps with motion sickness. You get motion sickness because your brain thinks you're moving, but your feet aren't. So, move your legs. Bounce them up and down. Alternatively, close your eyes.

I get horrific motion sickness playing Half Life 1 and 2, though. Couldn't play them for that reason alone. Only those games have done it to me.

  For any train journey i've ever had in China (dozens) i've found the best medicine is always readily available in the catering car, fairly inexpensively provided, and in bottled form, under the names Qingdao or Liquan. I highly recommend either one. They won't necessarily keep you from throwing up, you just won't mind so much if you do.

https://www.nbcnews.com/mach/science/look-sky-it-s-400-mph-train-ncna891771?icid=related

This might make you air sick at the same time

https://www.nbcnews.com/mach/science/jet-pack-lets-you-live-your-iron-man-fantasies-you-ncna893661

THIS IS THE WAY TO TRAVEL