Is this specific to Hull Trains? Managers are sometimes RMT or ASLEF if they were previously traincrew or other frontline roles. Otherwise, they’re normally TSSA or not unionised.
At Hull Trains they can be a member of any union they want but, like other TOCs, only certain unions have negotiating rights for specific grade groups.
When it comes to a disciplinary, anybody can advocate on behalf of the member of staff. It is usually, in the initial stage, the staff rep for the grade group but if it gets serious, the local union rep usually gets called in to take it on.
I was a TSSA rep in a maintenance depot and had to help NUR clerical staff on the odd occasion and my workshop rep colleague was NUR but “day to day” he looked after staff who were in all the “Confed” craft engineering unions.
Changing union in those days was not allowed and only when you got to senior management were you told to join the BTOG. I know a few managers who still stayed (on the quiet) in their previous union and had dual membership!
Today there are a lot of staff who are not members of a union. My plea to them is to join up, it’s the cheapest railway employment legal insurance & help you will ever get. And when you need it, you need it fast. Today (in old BR money) I’m a SM grade but still a TSSA member and will remain one.