The way it went with the case Im familiar with, company asked for a sick note after what they considered excessive use (4 times in a rolling 12 month period). The guy said in his state thats not a legal request, so they fired him for sick abuse.
Union grieved it, grievance obviously denied. Went to arbitration and arbitrator said that under RLA unless you can prove the company is wrong, their position is always correct, and with no sick notes he had nothing to counter. Arbitrator held the termination.