144 hour visa limits by train

We're making plans now and are wondering about the limits of the 144 hour visa for Shanghai. We're Dutch, currently in South Korea as tourists, and would fly to Shanghai from either Korea or Japan. Is it correct that we would then be able to exit by train to Hong Kong? And, if so, I understand that the 144 hour clock starts at midnight the day after we arrive, but when does it end - when we board the train for Hong Kong or when we cross the border into Hong Kong?

Thank you,

HF