Heavy rail equipment won't fit on its own trucks, but remember this is a streetcar tunnel, so light rail equipment would most certainly fit. The way this has been set up now, however, has the static displays built into place and with the facade on the portal you're not going to get anything bigger than a truck (or a bus, I suppose) into the tunnel.
Moderator: Massachusetts Bay Transportation Authority Avatar:3679A (since wrecked)/3623B (now in service as 3636B).
Waking up this old thread, Channel 4 (WBZ-TV) had a piece shot inside this station during a recent training exercise on the 6PM news. I imagine it will be on their later newscasts tonight (WSBK at 8, WBZ at 11) and maybe on their website, too, if you want to see what it looks like.