A company being consolidated, merged, or acquired into another company that is represented by a union under the RLA has a limited window to claim a representational dispute the consolidation, mreger, or acquisition might generate. Section 19 of the National Mediaton Board (NMB) Representation Manual deals with this very matter. The incumbant union does not automatically gain the representational rights to the new group, unless the company does not claim a dispute. Union's hope the company doesn't know this, and it has been used to gain representational rights over a larger class and craft of employees than existed with previous certification.
For instance, a company that has a union that only had 100 employee in the class and craft certified by the NMB because 51 of the 100 voted in a union, can automatically claim the certified representation rights to 1000 more class and craft employees that company acquires if it does nothing.
Of course, that would not be the actual will of those being unionized!
So, when you are warned "be carefule who you talk to", keep in mind, it works both ways.
See Section 19 to the NMB Representation Manual: