Motion sickness while using a laptop on a train is relatively common, especially for people who are sensitive to sensory conflicts. As your body feels the train's movement, your eyes stay focused on a non-moving screen, leading to symptoms like nausea or dizziness.
Sitting in a forward-facing seat and taking breaks to look outside can help alleviate symptoms. You can also use Carsick.app, an app that syncs your MacBook’s visuals with the train’s movement to reduce the conflict that causes motion sickness.