A military rocket train was propelled down a track at nearly nine times the speed of sound yesterday, thereby reaching a world land speed record for rail (though Guinness wasn't invited). The previous record was held by another rocket sled at about mach 8.5. Approximately three miles of the test was inside an inflated helium tunnel because, as one official said, hydrogen would be "a bad choice." The payload of this particular sled was classified, though only projectiles for a rail gun or some sort of warhead make any sense to us.
All of this was carried out at the Hollomon Air Force Base High Speed Test Track in New Mexico, where the 846th Test Squadron does their thing. Apparently, the test track can go up to mach 12, though they haven't thought of a reason to go that fast... yet. [The Register]
(Photo Courtesy Of Air Force, Previous Rocket Test)