What does your contract say? Does the contract say that the company awards trips based on seniority or not?
A grievance rep can only challenge the interpretation of an existing collective bargaining agreement. What does the agreement say? If it does not address the question at hand then you have no recourse.
If the contract states that schedules are awarded in a seniority order you have a valid grievance and furthermore your rep is required to forward it. Because the law allows only one bargaining agent and because that entity is the exclusive bargaining agent then that agent, ALPA, owes a duty of fair representation to its members.
The Railway Labor Act says that the parties shall make and maintain agreements on rates of pay and working conditions.
You have a valid grievance if the contract states that schedules are awarded in seniority order.