relief from overcrowding for sure. I'm happy about it!
I've been there several times - including staying at the Staybridge once which is behind the stadium - and have never seen such a tunnel.
There's a road tunnel under the railway tracks, and a long hallway going south from the ICC right next to it, but both end before the street that runs crosswise between the ICC and the stadium; you have to go outside to cross this street and the plaza in front of the stadium...which is no big deal, really; it's not that far.
I've never been to GenCon... and even though I want to, I'm nervous about it now because of the housing situation.
I went to Gen Con this summer and while I am happy I finally went I don't believe I would go again. The convention, in my mind, is VASTLY better when you are within walking distance of the action. I can check this off my nerd list underneath visiting Games Workshop's Nottingham HQ and meeting Margaret and Tracy. I recommend that people go if they haven't but once I got over the sheer number of people and the size of the main hall I left slightly underwhelmed.