Train prep's can get carried out at any location where a train might stop, and depending on access depends on how much of the prep is carried out. I believe there is also differences between a prep being carried out by a fitter and by a driver. Even on depot's a 'full prep' might not be carried out due to specific berthing arrangements (i.e. limited access down one side of a unit). In general, if a prep can be carried out in a station, and there is no obstructions preventing a wrong side release, then there is no reason why it can't be carried out in a safe manner, however, there may be rules that exist as a 'just in case' and I would always walk through a unit on a platform before releasing any doors to ensure that I had control of the situation.