I don't think this is unreasonable at all - working people cannot continue to fund pensioner's every whim!
If you are retired, why are you in a rush to get to places? You have all the time in the world! If you are in a rush a need a faster journey, perhaps you should be paying for that extra service.
Perhaps the extra rail fares could be paid out of the gold plated, triple locked pension increases.
This wont be a popular opinion, but in an aging population, I am afraid that pensioners have to start paying their way. And no, they didn't "pay in for it when they were at work" they paid for the previous generation of pensioners concessions, during a time when the ratio of working age people paying taxes to pensioners was far smaller. Perhaps they should have fixed the system for the future then, when they had the chance. It wasn't like the demographic time bomb wasn't very obviously ticking in the corner for the past 30 years.
That said, I wouldn't object to a voluntary top up to add rail fares or pensioners being charged a concessionary rate as has been mentioned above.