29 CFR 1202.4: Secret ballot.

In conducting such investigation, the Board is authorized to take a secret ballot of the employees involved, or to utilize any other appropriate method of ascertaining the names of their duly designated and authorized representatives in such manner as shall insure the choice of representatives by the employees without interference, influence, or coercion exercised by the carrier. Except in unusual or extraordinary circumstances, in a secret ballot the Board shall determine the choice of representative based on the majority of valid ballots cast.