(a) General rule There is hereby imposed on every rail employer for each calendar month an excise tax, with respect to having individuals in his employ, equal to 4 percent of the total rail wages paid by him during such month. (b) Tax on employee representatives (1) In general There is hereby imposed on the income of each employee representative a tax equal to 4 percent of the rail wages paid to him during the calendar month. (2) Determination of wages The rail wages of an employee representative