I joined in the 1970’s when it was still a closed shop - everyone had to join a union, without exception. At KX, the salaried staff used to join TSSA, the footplate staff ASLEF and the conciliation staff NUR. On the M&EE workshop side, the fitters were AUEW, electricians EEPTU and the smiths were with the boilermakers. The workshop staff used to be represented nationally by the “Confed”, a grouping of those trades and others. I think you had to be a senior officer grade (i.e. higher than MS5) to get into BTOG.
Although some drivers depots were recognised as NUR (largely due to historical reasons) and if you joined the NUR in any grade, as you moved up or around, you tended to stick with them but poaching of members by another union was strictly prohibited by a TUC agreement (the Bridlington Agreement) and the NUR fell foul of this in the mid to late 1970’s and some members had to join or rejoin another union.
At KX the guards used to be paid out at the south end of the station. The drivers (& workshop staff) at the north end of west side offices. One pay day the south end pay point was out of action and management announced that everyone would get paid out at the north end. There was nearly a strike by the drivers who didn’t exactly help matters by singing “The guard is the man” while they were passing the queue of guards. Management had to set up an alternative pay point for the guards fairly quickly!
There was no love lost between ASLEF and the NUR. One day Sid Weighell was being interviewed for TV in Room 127 West Side KX and ASLEF men took turns in starting a Deltic that was sitting on 8! But whether you were ASLEF, NUR or TSSA, everyone hated the TGWU - they represented the bus and truck drivers and were very anti-rail.