(a) The term “carrier” 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.

(b) The term “carrier” shall not include any company by reason of its being engaged in the mining of coal, the supplying of coal to carrier where delivery is not beyond the tipple, and the operation of equipment or facilities therefor or any of such activities.