Carbon footprint of live events and festivals.

[richardash1981]

Great Western do it in Cardiff (and from other accounts with considerable organisation set up) for Rugby Internationals, which shows it can be done - if the will is there.
https://www.railadvent.co.uk/2023/08/great-western-railway-to-run-72-trains-for-cardiff-rugby-international.html

[J Pearce]

Sports has the advantage that generally most of the fans are coming from one location (the home of the away team) and most stadia are city centre based, so charter trains work well. Most festivals are on greenfield sites in rural locations, so a train means a change to a shuttle bus, and the attendees are coming from a wide range of locations.

If you're doing dedicated transport as a charter service it makes more sense for it to finish on the festival site, rather than dumping 500+ attendees off a train into a railway station to wait for shuttle buses. Thus, many festivals run buses/coaches from major city centres to their sites. However one I attended this year had to turn several of their coaches into minibuses due to low take-up. The fuss of getting into a city with camping gear etc. to get the coach meant that most folk went straight to the festival site instead.
Events with a younger age profile generally do better with coaches, as parents will drop them at the coach pickup rather than take them all the way to the event.

As a contrary to everything I've just said, they do bolster train services for some horse racing events, which don't have the home/away planning advantage that football/rugby has.