EE is very mild on the motion sickness, but just to be on the safe side, you should ask for the front row (this is very easy to do - they've set up a special little queue just for this. Ask when you get to the loading platform.)
Hubby has motion sickness issues. When we rode in the middle of the train, it wasn't awful for him, but he also didn't want to ride again that same day. When we rode in the front row, he had absolutely no problem at all.