There was a news item today that
MTR▸ /Crossrail have lodged an objection to the advertising screen around the MSG/Populous proposed ... arena at Stratford. I imagine this was a standard planning step for the advertising screens, which were not part of the original proposal.
More details in Wikipedia, if you've missed the story so far, and some
LED-less visuals on dezeen.

Now, this site is a cramped triangle with railways on all its sides. It also included as part of its justification that this makes it ideal for getting big crowds in and out. However, we've all seen how keen railways are to take on that function, even in the "good" old days (last year).
I've not been digging for the planning documents, which would be with the London Legacy Development Corporation. Hover, the GLA has of course got involved, and
I found this response of theirs that included reactions from
NR» and the operators. And guess what - they don't think they could manage! I do see the GLA's point that 300 events a year means they can't be called "exceptional", but evidently they are still not routine as evening crowd sizes. NR did say any lights or reflections mustn't interfere with their signal sightings, and that was before the LEDs were mentioned.