In the Class 166, opening the doors between the carriages will open the first one but not the second. The second one then has no collision detection, so you can walk straight through it. Talking about doors in the HST in the coach with the disabled toilet, there is an invisible wall where the vestibule door would be in normal Mark 3s. You have to wait for the 'door' to open before you can continue walking through the train.
The recent patch fixed the HST glitch, but not the Class 166 glitch. A simple fix would just be to open both doors at the same time.