Looks like they've picked up a national story as there's something similar in the London Evening Standard (
https://www.standard.co.uk/news/uk/rail-passenger-numbers-2050-prediction-railway-industry-association-b1139949.html).
A doubling in numbers is not implausible - if you take Bristol as a case, the cumulative effect of reopened lines, stations being built and higher frequency will soon add up, along with growth in longer-distance commuting a few days a week. Temple Meads at 8am is now the busiest I've seen it in a long time.
Similar story for Birmingham, Newcastle and others, whilst the Transpennine electrification should result in plenty of sparks effect growth.
However, even if you add up all the regional increases, it's probably a drop in the ocean against London, where Crossrail 2 and the West London Orbital could probably add a fair whack to that figure on their own.