In the event of immobilisation of a vehicle on a level crossing, are you obliged to warn the rail vehicle driver if it impossible to remove the vehicle from the level crossing?

Yes, this is an absolute duty of the driver. According to Art. 28 para. 4 of the Road Traffic Act, if a vehicle becomes immobilised on a crossing, it must be removed immediately. If this is not possible, action must be taken to warn the train driver of the danger to prevent a disaster.