tim wrote:

That's where the door will be when the train stops and thus
where you have to stand to get on the train first,
everybody then forms an orderly queue behind you. (And
unfortunately, yes I can remember when it was, exactly,
like that!)

Given that platform edge doors also show passengers/customers (select
preferred description) where they must stand to board the train it seems tot
me that marking the tunnel wall or platform edge is a much more satisfactory
method.