i work as a contractor for Net Work Rail and have asked the question before (got no reply ) the company i work for gets a licence to gas the warrens then they pump grout down,its like runny cement. Ive been thre first hand and told net work rail your wasting your time,but as allways they say its a humane way of controling them and stabalising the embankment.You end up with rabbits running out covered with grout because the the gas carn't get deep enough