No common carrier shall employ any person as:
(a) A locomotive engineer who has not had at least three years’ actual service as a locomotive fireman or one year’s actual service as a locomotive engineer.
(b) A conductor who has not had at least two years’ actual service as a railroad brakeman in road service on steam or electric railroad other than street railway, or one year’s actual service as a railroad conductor in road service.
(c) A brakeman who has not passed the regular examination required by transcontinental railroads.