Definition of "second man"

I should imagine that the smooth riding and the quietness of the diesel or electric cab, coupled with the effect on the eyes of endless successions of sleepers disappearing from sight immediately under the driver's eyes, might in time have a soporific effect, so that the company of a second man, who can assist in signal observations when he is not at work in the engine cab, seems highly desirable in such conditions. In most countries using these forms of power, a second man is provided for on any train on which the maximum distance between stops exceeds a certain predetermined distance or time.

1961 July, Cecil J. Allen, “Locomotive Running Past and Present”, in Trains Illustrated, page 401