On June 4 and June 5, they are having special events. They anticipate 50,000 to 100,000 visitors [one can travel N to S or S to N, so I guess not everyone will be going to Erstfeld (North end).]
It seems, you must still buy a ticket to participate.
I'm getting this info via
www.sbb.ch. As much as I'd like to participate, the crowds they anticipate are a big turn-off.
But, from August 2 to Nov. 27, they are running (except Mondays), special excursion trains (presumably no crowds), which will allow a stop in the tunnel to explore.
Any other info would be appreciated.
PS: I'll be in Switz. on the weekend of June 4 and again in late August.