The term employee as used in this part includes every person in the service of a carrier (subject to its continuing authority to supervise and direct the manner of rendition of his service) who performs any work defined as that of an employee or subordinate official in the orders of the Interstate Commerce Commission now in effect, and as the same may be amended or interpreted by orders hereafter entered by the Commission pursuant to the authority which is hereby conferred upon it to enter orders amending or interpreting such existing orders: Provided, however, That no occupational classification made by order of the Interstate Commerce Commission shall be construed to define the crafts according to which railway employees may be organized by their voluntary action, nor shall the jurisdiction or powers of such employee organizations be regarded as in any way limited or defined by the provisions of this Act or by the orders of the Commission.