The world's first space railway goes on its maiden run Monday. U.S. astronauts went on a spacewalk outside the international space station Sunday to prepare it to roll.

Space station ground controllers will test their ability to command a small flatcar to roll along tracks on a 13-meter girder that space shuttle Atlantis astronauts mounted on the station last week.

Mission official Ben Sellari said the unit, called the Mobile Transporter, will crawl as it moves along the truss at speeds merely a couple of centimeters or less per second. "The Mobile Transporter itself has a top end speed unloaded of one inch [2.54 centimeters] per second," he said. "When we're crossing across various truss joints where we have to be very careful. As we transition across that, we have to drop down to a speed of about .06 inches [0.15 centimeters] per second, so nothing happening very rapidly there."

Working outside the outpost Sunday, shuttle crewmen Steve Smith and Rex Walheim got the trolley ready for its trial run by releasing clamps that held it to the girder during its launch on the shuttle.

When the girder is expanded to its full 100-meter length on future shuttle visits, the railcar will haul the station's crane back and forth to various construction sites.

Before preparing the trolley, the two spacewalkers made electrical connections on the girder Sunday and ran cables through it that will eventually supply power and communications to the crane. Many of the cables contain hair-thin fiber optic strands that can break if handled wrong, causing some concern on the part of astronaut Smith. "These cables are again sticking together because of the tape that's used to hold it in the clamps," he said. "It's a little scary taking them apart because they are fiber optics."

Nevertheless, the two crewmen successfully mated 44 connectors without a problem.

Mission official Milt Heflin pronounced this third spacewalk of the shuttle visit a success. "It's been another great day in the construction business for the space station," he said.

An alternate pair of astronauts will finish configuring the girder complex during a fourth spacewalk on Tuesday.

(Space Station railcar test is scheduled to begin at 1129 UTC, Monday, April 15)