It's very frustrating for someone with UK English as a first language to have everyday expressions constantly rejected, and repeated pleas to have them accepted apparently ignored. I usually don't "eat anything" while "travelling on the Underground" in London.
I am Japanese and not good at English and Chinese. So, I will write in Japanese.
となると英文も"You are not allowed to..."が正しいのではないかと思います。
也是對的英語,但有不同的意思; for most people I think "on the subway" would mean you're in (riding on) one of the cars while "in the subway" would mean you're anywhere in the subway including the stations, walkways etc.