Live music/festival industry told to reduce climate change impact.

[whole diary]

[broadgage]

6 trains an hour... so passengers missing the first hour or standing round for an hour before the show starts?

Or the majority arriving in good time and presumably enjoying food, drink, and retail opportunities.
With good planning, 12,000 people can probably arrive by rail in less time than it takes for 10,000 cars (average 1.2 people in each) to negotiate roads and access routes and for the occupants to then walk from car park to actual venue.

Needs well planed infrastructure to handle 1,000 people from one train in not many minutes before the next train arrives with another 1,000.
Two platforms as a minimum each able to accept at least a 12 car train, 16 car would be better for future proofing. Wide enough walk ways to handle large numbers. Ample toilets, might need as many as 100 for each platform, remembering that after the show that three train loads of homeward bound passengers might be waiting. If drink has been taken, as many as 20% of those waiting may need the toilet. 600 wanting to go, or 6 per toilet to avoid excessive waits.
All this is entirely doable and no harder than huge scale car parking.
But it needs to be planned from the beginning. Platform length and number of platforms are expensive to alter later.