There are invisible walls improperly blocking access to subways between platforms at several stations - at least Preston Park and Battersea Park (preventing access to a platform with a poster collectible). Annoying because it means you have to unrealistically walk across the track to change platforms. Additionally it's painfully clear that the scenery was never finished in the subway at Purley Oaks (I think) which just opens out onto an undecorated grass area with no doorway etc.
Spawn into Battersea Park Station and will put you next to the poster and you will be able to get it.
