U.S. space shuttle astronauts took their second spacewalk in three days Monday, acting as orbiting mechanics at the International Space Station. They made repairs to the outpost's cooling system and its railroad flatcar.

Thespacewalkby shuttle Discovery astronauts Mike Fossum and Piers Sellers lacked the theater of their Saturday venture, when they bounced and twisted at the end of a 30-meter long pole like circus high-wire performers to test its stability as a platform to make shuttle repairs in orbit.

For this outing, they continued more mundane maintenance they began Saturday on the space station's rail car, which rolls back and forth to position the outpost's mechanical arm and heavy equipment during construction. They replaced a backup cable that delivers power, video, and data to the car. It had been inadvertentlyseveredby a cable cutter in December.

Shuttle pilot Mark Kelly, who supervised the spacewalk from inside, spoke to Fossum as the astronauts prepared to replace the reel from which the cable unwinds as the railcar moves.

Kelly: "You look pretty comfortable out there, Mike.

Fossum: "Well, I'm working up a sweat."

Sellers found the working conditions on this task a bitcramped.

"This is a tricky place to work," said Piers Sellers.

The two astronauts also replaced a spare external pump on the station's cooling system.

Sellers and Fossum have one more spacewalk on Wednesday. They will test techniques to inspect and repair the reinforced carbon panels on the front edges of the shuttle wings while in orbit. That was the part of the shuttle Columbia punctured by a piece of hardinsulating foamthat rushed down from the external fuel tank during its 2003 launch, dooming it to a fiery disintegration during its re-entry into the atmosphere.

Discovery's flight has tested changes the U.S. space agency, NASA, has made to the fuel tank to minimize the amount of threatening foam it sheds at launch. Ground controllers have used new in-flight inspection cameras and sensors installed since the Columbia accident to look over the orbiter's fragile heat shield. They have found no launch or any other damage that would prevent Discovery's safe return, so they have cleared it for re-entry.

Shuttle official Phil Engelauf says the successful Discovery mission so far and the refurbishment of the space station's railcar prepare the way for the resumption of station assembly, which has been stopped since Columbia's demise.

"We have a huge amount of work coming ahead and the success of this mission is a good indicator that we are heading toward another launch in the August time frame unless anything new comes up," said Phil Engelauf. "The assembly sequences is set to proceed fast and furious here. We're only six or seven weeks out from the next mission."

In the meantime, Discovery is due back on Earth next Monday to end a 13-day mission that has delivered cargo and a third crew member to the space station, German astronaut Thomas Reiter.