A receiver, trustee, or other individual or body, judicial or otherwise, in the possession of the property or operating all or any part of the business of a carrier, or of a company or person owned or controlled by or under common control with such a carrier, which operates any equipment or facility or performs any service in connection with the transportation of passengers or property by railroad, shall be deemed to be an employer beginning as of whichever of the following three dates is the earliest:

(a) The date that it takes possession of such property; or

(b) The first date on which it has authority to operate all or any part of the business of such a carrier, company or person; or

(c) The date that it begins operating without appointment or authorization all or any part of the business of such a carrier, company or person;

Provided, however, That the receiver, trustee, or other individual or body, judicial or otherwise, shall be an employer only with respect to such individuals as would be employees if the preceding employer had continued in the possession of the property or the operation of the business.