I'm a little worried by this:
. . . . That's what I'm getting at. Is there any flexibility in the railway requirements? And if not, why not?
Of course not. And it's not just on the railways, it's how we assess risk.
What we have learnt over decades of accidents, is not just the circumstances which prevail when accidents happen (and that is very useful knowledge in reducing the exposure to risk in future), but also the circumstances in which people's faculties in assessing risks becomes diminished.
As a result of that learning, we have introduced rules and regulations which aim to reduce the incidence of accidents by prohibiting the causal factors identified by that canon of evidence; that includes restrictions on the circumstances which are seen to have an adverse effect on a person's faculties in assessing risk. And that includes prohibition on a person in a safety critical role making a self-assessment of their abilities once they have crossed the theshold of the number of hours of continuous work which is deemed to be safe.
Or in simple terms, no, you don't ask someone who has already crossed the line into a danger zone to self-assess their own ability to be safe. That wouldn't be 'flexibility', it would be dangerous. That is comparable to asking a drunk if they feel able to take control of a hazardous system. It worries me that you appear not to understand how safety is achieved.