Got those in Luton as well, basically the reason behind it is they are 'normal' buses for most of the journey. Then on certain sections they've either taken prior rail track and modified them or built these new concrete things and as others have said they have guide wheels fitted to the sides to aid the steering. Why I don't know, maybe it is simply to prevent them crashing and blocking the route as the ones in Luton there is no easy way to get to them from the sides if there was a smash. There again maybe its just to give the lazy bloody bus drivers 20mins rest every hour or something lol
Happily collecting things all my life...