The train had been towed into the tunnel the previous night and used in an evacuation exercise, where some 300 volunteers (mostly British and German students) successfully evacuated it in four minutes. (Article with more details, in German.) From there it was towed along the high speed line (the train type does not yet have regulatory approval to run under its own power in Britain) and parked at St. Pancras for display. It was behind glass, in the secure area, and while journalists and VIPs (including, apparently, The Man In Seat 61) were shown around, the general public had to content themselves with viewing it through the glass wall, the train's red LED destination board tantalisingly scrolling destinations including Amsterdam, Cologne and Frankfurt.
Deutsche Bahn plan to start services through the tunnel in December 2013; that is apparently how long it'll take to get regulatory issues sorted out and a fleet of trains prepared. The service will run towards Brussels, where trains will split, with one half going up to Amsterdam via Rotterdam, and the other half going eastwards to Frankfurt. It'll be interesting to see whether this results in cheaper rail fares through the tunnel.