I don't have an answer to your question I'm afraid, but I have the opposite experience to you. I find the steady, rhymic sway of the train carriage quite pleasa

I don't have an answer to your question I'm afraid, but I have the opposite experience to you. I find the steady, rhymic sway of the train carriage quite pleasant and is likely to make me nod off. It's not just me, other passengers can often be seen dozing. Even a jolt or bump in the journey doesn't appear to disturb some passengers while dozing. Someone unknown once likened the steady sway of the train to like rocking a cot from one side to another. Could that be why it makes some passengers sleepy? (I have no idea!)