No, I am not saying that my track is so rough that one wheel is lifting as you describe. If it were, then I would be relaying it as everything would derail. The problem had to be something unique to Hornby Gresleys as they are the only carriages with the problem. I have not yet bought any Pullmans because of this fault on the Gresleys and so Hornby should take note.
What appears to have been happening is that, in particular places, the top of the bogie frame was catching on the bottom of the solebar (the framework supporting the body sides). This happened as the carriage progressed through the crossings in the pointwork, the body rocking and twisting and the bogies canting and turning as they are supposed to.
The effect of the bogie frame catching in this way is to momentarily limit its movement, consequently the wheels are no longer free to run parallel to the rails and the flange on one wheel is pressed against the rail, "looking", as it were, for an opportunity to climb out of the 4 foot way. This opportunity could be offered by a rail joint, a crossing or the sharp end of a point blade. Then the carriage derails.
The Kadee couplings helped to a great extent by ensuring that the point of attachment of the carriage to its neighbours is along the centre line of the vehicle. With a tension lock coupling, this is not the case because because at any moment only one or other of the two hooks may be in contact with the bar of the coupling on the attached vehicle. Then the pull is not central to the vehicle, the bogie is pulled to one side or other of the 4 foot and a wheel flange is pressed against the rail looking for an opportunity to climb out.
When I fitted Kadees to the Ian Kirk kit built carriages in place of the old tension locks, their running much improved, but it was not the cure for the Hornby Gresleys.
Could it possibly be that the Chinese manufacturers test their Hornby stock on ideal track laid on flat boards without any underlay or ballast, so they would not get the problems that modellers in the "real world" experience? I am not having any problems with their new Stanier stock, it runs perfectly and so perhaps they have now identified the cause of the problem and cured it.
Please, if you have been having similar problems, try fitting my washers as described in my previous posting and report the outcome.