A "furious complaint" doesn't deserve more than an acknowledgement.
No matter what the circumstances, civil and dignified is the way to go about things.
You are quite simply nothing other than incorrect.
If something happens three times, and you write three letters of complaint each pointing out what has happened before and what hasn't been done, and if you get three completely identical replies, then that lets you know that someone isn't even as much as bothering to read your letters or that they are and that they do not care.
If that happens you need to step up the pressure. It may have worked as it got a reply that was different, but could we take the chance of trying it and finding out?
No. For us the system has been brought down by arrogant people that think they know better and that will abuse a sick pensioner because they can't see what's wrong with her and because she looks more like 50ish.
The fact that this can keep on happening and that nobody seems to care about it, and that a guard that comes across a passenger (with a disabled pass) that has missed the stop where they should have been helped to change train and is heading off into the wrong part of the country will do little more than abusively accuse them of faredodging, tells me that there is a deep seated attitude problem. It seems to me that people have developed a culture of routinely deciding they know best rather than providing the service they are supposed to be.
I don't doubt that there are people that are abusing the system; but if staff decide to take a matter they know nothing about into their own hands they'll routinely get it wrong.