(a) The term “employer” means any carrier (as defined in subsection (b) of this section), and any company which is directly or indirectly owned or controlled by one or more such carriers or under common control therewith, and which operates any equipment or facility or performs any service (except trucking service, casual service, and the casual operation of equipment or facilities) in connection with the transportation of passengers or property by railroad, or the receipt, delivery, elevation, transfer in transit, refrigeration or icing, storage, or handling of property transported by railroad, and any receiver, trustee, or other individual or body, judicial or otherwise, when in the possession of the property or operating all or any part of the business of any such employer: Provided, however, That the term “employer” shall not include any street, interurban, or suburban electric railway, unless such railway is operating as a part of a general steam-railroad system of transportation, but shall not exclude any part of the general steam-railroad system of transportation now or hereafter operated by any other motive power. The Surface Transportation Board is hereby authorized and directed upon request of the Railroad Retirement Board, or upon complaint of any party interested, to determine after hearing whether any line operated by electric power falls within the terms of this proviso. The term “employer” shall also include railroad associations, traffic associations, tariff bureaus, demurrage bureaus, weighing and inspection bureaus, collection agencies, and other associations, bureaus, agencies, or organizations controlled and maintained wholly or principally by two or more employers as hereinbefore defined and engaged in the performance of services in connection with or incidental to railroad transportation; and railway labor organizations, national in scope, which have been or may be organized in accordance with the provisions of the Railway Labor Act [ 45 U.S.C. 151 et seq.], and their State and National legislative committees and their general committees and their insurance departments and their local lodges and divisions, established pursuant to the constitution and bylaws of such organizations. The term “employer” shall not include any company by reason of its being engaged in the mining of coal, the supplying of coal to an employer where delivery is not beyond the mine tipple, and the operation of equipment or facilities therefor, or in any of such activities.