British Transport Police intelligence often results in the deployment of buses and sometimes even additional trains. Presumably they get a lot of intelligence from the local force and the event promoters who know how many tickets are sold.
It's also common that trains to and from the football are specially protected from cancellations due to shortages of units or staff, even if it means others are left with longer waits.
I see it's the Grand National this weekend. On Saturday there are an absolutely enormous number of extra trains between Liverpool and Aintree, and some of the Ormskirk services are likely to be longer than usual. There are also two extras each way between Preston and Ormskirk.
As mentioned in my earlier post, there are liaison officers who find information about the plans of groups of supporters. There is a lot of information that can be gained from having an informal relationship with regular travellers. They learn things like the pubs people plan to go to, the places that people want to avoid, and the timings. As mentioned, the pub I was going to in a particular place was probably a signal of where did not need police because no "youth" would be going there.
Things like Internet Forums played a part back in the 2000s, so I am sure they are big part now (alongside X, Facebook, Insta etc.)
