49 USC 28301 - General

(a) Eight Hour Day.— 
In contracts for labor and service, 8 hours shall be a days work and the standard days work for determining the compensation for services of an employee employed by a common carrier by railroad subject to subtitle IV of this title and actually engaged in any capacity in operating trains used for transporting passengers or property on railroads from
(1) a State of the United States or the District of Columbia to any other State or the District of Columbia;
(2) one place in a territory or possession of the United States to another place in the same territory or possession;
(3) a place in the United States to an adjacent foreign country; or

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(4) a place in the United States through a foreign country to any other place in the United States.
(b) Application.— 
Subsection (a) of this section
(1) does not apply to
(A) an independently owned and operated railroad not exceeding one hundred miles in length;
(B) an electric street railroad; and
(C) an electric interurban railroad; but
(2) does apply to an independently owned and operated railroad less than one hundred miles in length
(A) whose principal business is leasing or providing terminal or transfer facilities to other railroads; or
(B) engaged in transfers of freight between railroads or between railroads and industrial plants.