In the private sector, "supervisors" can be in unions as long as they don't meet the tests of a 2(11) supervisor (i.e., the right to hire, fire, promote, change working conditions, etc.). At least that's the case with the NLRA. I don't know the finer details of how the RLA defines supervisors. That's why foremen can sometimes be in unions - they don't always meet the standards test with 2(11) status. In the past, employers have tried to get nursing unions decertified, alleging that nurses have 2(11) status because they oversee nurse aides and other similar employees. The NLRB has generally ruled that nurses don't meet the qualification for 2(11) status.
Now the public sector, which doesn't fall under the jurisdiction of the NLRA (with the notable exception of the USPS) is a different animal, and managers who would hypothetically fall under 2(11) status in the private sector can oftentimes be union employees.