Any municipal corporation which has heretofore or shall hereafter acquire from a private owner any urban transportation system which at the time of such acquisition has or had in effect any pension or retirement system for its employees, shall assume all such obligations with respect to continued contributions to and/or administration of, such retirement system, as the private owner bore or shall bear at such time, insofar as shall be necessary to discharge accrued obligations under such retirement system to beneficiaries who are not thereafter made members of a municipal or state retirement system.