law: Railways Act, 1989
  27. Use of rolling stock. 
  A railway administration may use such rolling stock as it may
  
  consider necessary for the construction, operation and working of a railway:
  Provided  that before using any rolling stock of a
  
  design or type different from that already running on any section of the railway,
  
  the, previous sanction of the Central Government shall be obtained for such
  
  use.
  Provided  further that before giving any such sanction,
  
  the Central Government shall obtain a report from the Commissioner that he has
  
  made a careful inspection of the rolling stock and, in his opinion, such
  
  rolling stock can be used.