oh yes, accessable to all, but with severe restrictions. I don´t know what exactly this could be yet, but it will be fun to find out. Reduced speed for sure, maybe reduced visibility, accuracy or moral.
I also want to let the player feel how confined this space is by making it unable for characters to pass each other - maybe even blocking the entire tunnel when somebody (or something) dies - which answers your "tunnel rush" question I think - when only one soldiers at a time drops out of the tunnel they will be easy targets. Might be funny to drop a grenade tough