29 C.F.R. § 1205.2
The provisions of the third, fourth, and fifth paragraphs of section 2 are by law made a part of the contract of employment between the carrier and each employee and shall be binding upon the parties regardless of any other express or implied agreements between them. Under these provisions the employees are guaranteed the right to organize without interference of management, the right to determine who shall represent them, and the right to bargain collectively through such representatives. This section makes it unlawful for any carrier to require any person seeking employment to sign any contract promising to join or not to join a labor organization. Violation of the foregoing provisions is a misdemeanor under the law and subjects the offender to punishment.
29 C.F.R. § 1205.2