With Bristol City regularly getting crowds now in the 20k and the rugby 7k it is mind blowing that GWR▸ haven't looked into this more as the potential customer base to tap into is massive
... and - taking this discussion even further off-topic (and I can, because I'm an admin!
) - the incredible amount of road traffic on the A369 from Portishead into Ashton Gate, Bristol, on football / rugby days / evenings also shows how the reopening of the Portishead Branch Line for passenger services would be
so very useful.
CfN
The problem you have taking big 'event' crowds is that you need a lot or resource for very rare use. You can cover up to the amount of rolling stock you use on a weekday peak perhaps, but that's unlikely to be anything like enough, especially as the rail industry and working hours soften that peak from what it used to be.
I am minded of a web site visitor graph from a few years back - our web site when we appeared on TV. If we had to pay for a fixed flat line resource, just look at what a huge area (cost) would be under that line, but above the actual!