Existing transportation systems—Cases in which no transportation employees, or only certain employees, are covered

Service performed in the employ of a State or political subdivision in connection with the operation of its public transportation system shall not constitute covered transportation service if—

(A) any part of the transportation system was acquired from private ownership after 1936 and prior to 1951, and substantially all service in connection with the operation of the transportation system was, on December 31, 1950 , covered under a general retirement system providing benefits which, by reason of a provision of the State constitution dealing specifically with retirement systems of the State or political subdivisions thereof, cannot be diminished or impaired; or

(B) no part of the transportation system operated by the State or political subdivision on December 31, 1950 , was acquired from private ownership after 1936 and prior to 1951;

except that if such State or political subdivision makes an acquisition after 1950 from private ownership of any part of its transportation system, then, in the case of any employee who—

(C) became an employee of such State or political subdivision in connection with and at the time of its acquisition after 1950 of such part, and

(D) prior to such acquisition rendered service in employment (including as employment service covered by an agreement under section 218 of the Social Security Act) in connection with the operation of such part of the transportation system acquired by the State or political subdivision,

the service of such employee in connection with the operation of the transportation system shall constitute covered transportation service, commencing with the first day of the third calendar quarter following the calendar quarter in which the acquisition of such part took place, unless on such first day such service of such employee is covered by a general retirement system which does not, with respect to such employee, contain special provisions applicable only to employees described in subparagraph (C).