Has anyone ever had a satisfactory reason as to why it's supposedly commercially sensitive?
Such information only appears to have become so highly commercially sensitive around the time that HS2 popped its ugly mug up, with varying and at times dubious looking passenger figures spouted here and there.
Is there really a correlation between 'commercially sensitive' and the advent of HS2?
How many people use the railways on a daily station by station and station-pair route by route basis, is of paramount public interest given the amount of money the taxpayer and fare-payer pay up for the system. They have a moral right to know whether that money is providing proper value, and without being able to ascertain actual performance vs numbers that isn't possible.
And when billions are proposed to be spent on railway work on the basis of supposed demand even more so.
There's no credible commercial reason for keeping this basic information hidden and top secret, and the secrecy and lack of transparency when so much money is changing hands is an affront to the notion that the government is securing best value for the taxpayer. We should never have to take politicians at their word and hand over our money without question.
Passenger flows for each individual station are well known and are published on a regular basis by ORR.
Station-pair flows are not generally published - partly due, I would suggest, because of the enormous number of station pairs making the data set unwieldy. There are also inaccuracies because of the grouping of stations and the use of season tickets and similar which are sometimes difficult to allocate to a specified pair of stations.
Because of the these inaccuracies due to the very nature of the ticketing system at one of my local 'Meet the Manager' events it was said that the only really accurate figures are obtained from interviews of passengers on trains or at stations.
Anyway detailed information on the number of passengers making up any particular station to station flow gives no information about the
revenue for that flow as ticket prices vary and the revenue will depend on the mix between full price, season tickets, child tickets, reduced price off-peak tickets and any other special offers.
This, I suspect is the information you really want. It is the reason BR developed ORCATS.