The simple answer to disappearing trouble cards is to arrange with the union to keep a log of defects reported so that it can be bought to managements or other authorities attention.
I remember in the 60's that there was an ongoing dispute at Upminster where the depot staff just got worse and worse with the maintainance of stock. The Drivers/Guards just made a note in there notebooks and gave the union reps the details. Other depots on line then started to do likewise. The upshot was that trouble cards started to be date stamped and numbered and maintainance dramatically improved.
After a while it was found that one particular car (21120) had been reported so many times that management ordered a drastic solution. The fault was a rattle of something loose on the leading truck which clattered and banged a lot in motion. I ALWAYS had the damn thing OOS as I thought it was dangerous and had passengers complain about the noise too.
So what did they do? Swapped the offending truck with one under a middle ndm so that a driver could not hear it. As for this piffle that a train cannot be tested on the road there used be paths in the WTT that were designated "test run" whereby depot staff could accompany an empty train on the road to verify a defect. Something else that has gone by the board I suppose.
If management are so insistent that employees do their job by the rules then drivers should be equally insistent that depot staff do THEIRS. Staff in depots have been getting away with this for far too long and either not repairing said defect or leaving it to the "call point men" on the road.