29 CFR 1206.2 – Percentage of valid authorizations required to determine existence of a representation dispute
(a) Upon receipt of an application requesting that an organization or individual be certified as the representative of any craft or class of employees, or to decertify the current representative and have no representative, a showing of proved authorizations (checked and verified as to date, signature, and employment status) from at least fifty (50) percent of the craft or class must be made before the National Mediation Board will authorize an election or otherwise determine the representation desires of the employees under the provisions of section 2, Ninth, of the Railway Labor Act.
(b) Any intervening individual or organization must also produce proved authorizations (checked and verified as to date, signature, and employment status) from at least fifty (50) percent of the craft or class of employees involved to warrant placing the name of the intervenor on the ballot.