Mick,
for the later Bachmann coach releases ordinary quality styrene glue works perfectly. For early coaches from the 1990's I have a suspicion that they are not glue friendly styrene as I remember problems with glued joins not adhering correctly even though well reinforced. Perhaps they are an ABS compound. If you have early coaches then perhaps experiment with styrene glue.
For all my recent bashes (numerous others in a state of limbo remain 'undiscovered') I used the mid-2000's release Big Hauler coach kits as they are unpainted and glue friendly. I get an idea in my head and feel the need to build it. Once the construction stage is done then boredom sets in. At present I am painting two D&RGW coaches (Combine #127 and RPO #340 at scale length). I built these about five years ago and felt guilty seeing them on a shelf unloved.