I did hear about a couple of bus drivers challenging false positives for cocaine, they took hair tests at their own expense and these showed they had no trace of the substance over a period of a few months (including the period of the false positive). Both drivers took their employers to an ET for unfair dismissal and won on the grounds that the tests had been contaminated by cocaine present in the banknotes they handled as part of their job. Quite how long back a hair test will cover I don't know.
If this route fails, then I think the only case would be the fact that the failure to notify the OP meant that it wasn't possible to get a second sample tested. AFAIK, the correct protocol is that they split the sample provided into two, so that if the first one comes back positive, they can send the second one to a lab of the subject's choice.
The only time of the railway I've heard of an allegation of inaccurate testing was the Northern Line driver who was dismissed in 2015 after providing a positive breathalyser test, it was suggested that him having diabetes caused him to fail and resulted in a successful strike ballot. It all went a bit quiet after that so I'm guessing a settlement was reached or he was reinstated, otherwise I'd have thought it'd have got to an ET.
My advice for anyone working on the railway is that you avoid illegal drugs/"legal highs" like the plague and that you avoid mixing socially with people who you know to use them.