I wasn't familiar with this dataset, apart from the top 10most crowded trains, so I did spend a bit of time yesterday looking at the methodology.
The statistical release has some information starting here:
Data source and coverage
This publication focuses on passenger numbers during the morning and evening peak hours on Tuesday, Wednesday and Thursday, when rail travel tends to be busiest. The statistics are based on passenger counts carried out at specific station stops between 16 September and 13 December 2025, and represent travel on a typical autumn weekday. As a result, they do not represent all rail journeys.
There is also more detailed notes and definitions documentation which is here:
www.gov.uk
This document provides a range of background information on the Department for Transport’s (DfT) rail passenger numbers and crowding statistics. It also details some of the factors that may affect the accuracy of the statistics.
The data appears to be taken at terminus stations and a few selected stations eg Vauxhall (but how these stations are selected, and others just before interchanges where many alight aren't selected, is not explained). The choice of these extra stations appears to be be historic from years ago and thus does not include busy sections of line not near a city terminus.
The London stations are shown on a map in figure 4.
It is also my impression that what is included is historical, with the major changes being inclusion of various big cities outside London from 2010, plus Brighton, Cambridge and Reading added in 2017. In London, using zone 1 as a boundary looks much more questionable in 2025 than it would have done many years ago.
An obvious one is GEML outer services. With five major options to transfer to at Stratford westbound, I wonder how load factors east of Stratford look?
Similarly, didn’t C2c have to address some severe loadings between Barking and West Ham?
And most Lea Valley peak trains are most crowded north of Tottenham Hale. Looking at the examples, it is quite challenging to come up with a simple rule of what should be in and what should be out.
This isn't just a London issue. Wearing my Fenland hat, I am surprised that, when Cambridge was added, it did not not also include Cambridge North, and I wonder what will be done in autumn 2026 now that Cambridge South is open.