Section 173-F. Every terminable permit, and every amendment to any existing license, grant, franchise, permit or consent incorporating therein the terms of a terminable permit, shall contain an irrevocable option for the purchase by the city, town or village, either directly or through its nominee or nominees, of all or any part of the property, plant and equipment of the street surface railroad at any time, at a price agreed upon or to be ascertained and determined in a manner agreed upon at the time of the granting of the permit or amendment and to be set forth therein

Any  such  permit  or  amendment  may  contain  provisions  for
  amortizing  out  of  earnings  or  in  accordance  with  an amortization
  schedule or schedules to  be  set  forth  therein,  over  a  period  not
  exceeding  fifty years, the price to be paid for all or such part of the
  property, plant and equipment of the street surface railroad and for any
  additions thereto or betterments or improvements thereof, so  that  upon
  the  exercise of the option the city, town or village, or its nominee or
  nominees, shall be required to pay only the unamortized portion of  such
  price,  and  may  contain  any other terms, agreements, stipulations and
  conditions as may be authorized by any statute of the state, or  as  the
  city,  town  or  village may deem proper and in the public interest, and
  may provide for the termination thereof for any legal cause.