If any person by unlawful act or by any wilful omission or neglect or by tampering with safety devices, endangers or causes to be endangered the safety of any person travelling on or being upon any metro railway, or obstructs or causes to be obstructed or attempts to obstruct any rolling stock upon any metro railway, he shall be punishable with imprisonment for a term which may extend to seven years:
     Provided that, in the absence of special and adequate reasons to the contrary to be mentioned in the judgment of the court, such imprisonment shall not be less than--
         (i) six months in the case of first conviction; and
         (ii) two years in the case of conviction for the second or subsequent offence