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Passenger kilometres are a relatively new statistic, and when first published (intermittently) as passenger miles in the 1930s they were estimates of one month's traffic. From 1948 the British Transport Commission and then the British Railways Board continued to provide estimates (no longer apparently based on a single month), and modern totals for passenger kilometres are included by the ORR in National Rail Trends.
Even though they were estimated for many years, passenger kms provide an alternative indication of the level of rail use. The largest total on record for a long time was 56.7 billion (1945), but this was overtaken by the figure for 2011-12, which reached 57.3 billion (there had been at least one 'change of series' after 1945, but the effect was not apparently significant).
A further key statistic is the length of an average journey, which has tended to grow since the 1930s. It seems unlikely that the 1945 passenger kms total was reached in any year before then, because the journey totals (which are known for most years since 1842) were not high enough when combined with significantly shorter average journeys. We are continuing to research these figures and will update the tables when we can trace more or better data.
For consistency's sake, where necessary we have converted the former measure of passenger miles to passenger kilometres.
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