A windowless vehicle makes it worse, because you don't see any movement, you just feel it. As klibertp said....
> Motion sickness comes from the conflict between your eyes and the part of your ear which tells you which way is up and lets you stay in balance.
If there's no windows, then your eyes will say "We're not moving", but your inner motion sensor in your ear will say "We're definitely moving", and you'll create motion sickness.
It's for the same reason some people feel motion sickness in some VR apps that involve movement in the virtual world that doesn't match your real world movement, such as driving a car in VR. Your eyes say "We're moving", but your ear says "No we aren't!", and the discrepancy triggers motion sickness.
> Motion sickness comes from the conflict between your eyes and the part of your ear which tells you which way is up and lets you stay in balance.
If there's no windows, then your eyes will say "We're not moving", but your inner motion sensor in your ear will say "We're definitely moving", and you'll create motion sickness.
It's for the same reason some people feel motion sickness in some VR apps that involve movement in the virtual world that doesn't match your real world movement, such as driving a car in VR. Your eyes say "We're moving", but your ear says "No we aren't!", and the discrepancy triggers motion sickness.