U.S. astronauts embark on another spacewalk Saturday to continue outfitting a massive external girder mounted to the international space station. Their task includes wiring a cable to the world's first space railway.

Two space shuttle Atlantis crewmen will make the second of four spacewalks outside the station to continue bolting down the 13-ton girder. An alternate team fastened it on one side after the station's crane mounted it Thursday.

It is the centerpiece of a truss that will ultimately span 100 meters to support solar energy panels and cooling radiators for future research laboratories.

The two crewmen will also connect cables to power a railway flatcar attached to the girder. The rail car will ferry the station's crane across the completed span during future construction efforts.

U.S. mission official Ben Sellari says ground technicians have begun activating computers, navigation equipment, and other systems on the new girder, which the space agency has designated S-Zero. "So, all in all, over the last 24 hours or so, S-Zero has come to life on orbit as is looking very, very good," he added.