Elephant & Castle on Platform Three has an invisible wall towards the far end of the platform meaning you can't walk to the end of the platform and get the monitors achievement, or change ends on the platform rather than through the train.
I don't want to keep adding threads so I'll add this here as well - driving on the Bakerloo line and my doors are opening and closing automatically as if I have a guard
Yeah I worked around it by walking down on my own train, just pointing it out though it shouldn't really be there haha