First. Submission of controversy to arbitration

Whenever a controversy shall arise between a carrier or carriers and its or their employees which is not settled either in conference between representatives of the parties or by the appropriate adjustment board or through mediation, in the manner provided in sections 151—156 of this title such controversy may, by agreement of the parties to such controversy, be submitted to the arbitration of a board of three (or, if the parties to the controversy so stipulate, of six) persons: Provided, however, That the failure or refusal of either party to submit a controversy to arbitration shall not be construed as a violation of any legal obligation imposed upon such party by the terms of this chapter or otherwise.

Second. Manner of selecting board of arbitration

Such board of arbitration shall be chosen in the following manner:

(a) In the case of a board of three the carrier or carriers and the representatives of the employees, parties respectively to the agreement to arbitrate, shall each name one arbitrator; the two arbitrators thus chosen shall select a third arbitrator. If the arbitrators chosen by the parties shall fail to name the third arbitrator within five days after their first meeting, such third arbitrator shall be named by the Mediation Board.

(b) In the case of a board of six the carrier or carriers and the representatives of the employees, parties respectively to the agreement to arbitrate, shall each name two arbitrators; the four arbitrators thus chosen shall, by a majority vote, select the remaining two arbitrators. If the arbitrators chosen by the parties shall fail to name the two arbitrators within fifteen days after their first meeting, the said two arbitrators, or as many of them as have not been named, shall be named by the Mediation Board.

Third. Board of arbitration; organization; compensation; procedure

(a) Notice of selection or failure to select arbitrators

When the arbitrators selected by the respective parties have agreed upon the remaining arbitrator or arbitrators, they shall notify the Mediation Board; and, in the event of their failure to agree upon any or upon all of the necessary arbitrators within the period fixed by this chapter, they shall, at the expiration of such period, notify the Mediation Board of the arbitrators selected, if any, or of their failure to make or to complete such selection.

(b) Organization of board; procedure

The board of arbitration shall organize and select its own chairman and make all necessary rules for conducting its hearings: Provided, however, That the board of arbitration shall be bound to give the parties to the controversy a full and fair hearing, which shall include an opportunity to present evidence in support of their claims, and an opportunity to present their case in person, by counsel, or by other representative as they may respectively elect.

(c) Duty to reconvene; questions considered

Upon notice from the Mediation Board that the parties, or either party, to an arbitration desire the reconvening of the board of arbitration (or a subcommittee of such board of arbitration appointed for such purpose pursuant to the agreement to arbitrate) to pass upon any controversy over the meaning or application of their award, the board, or its subcommittee, shall at once reconvene. No qu