Circling Earth aboard the International Space Station, the car on this railway will have a top speed of only 300 feet per hour, but the entire line -- tracks and all -- will travel almost nine times faster than a speeding bullet, over 17,000 miles per hour, in orbit. The rail line eventually will stretch almost 100 yards along the structural backbone of the station, serving as a mobile base from which the station's Canadian-built robotic arm can assemble and maintain the complex.

"To build the rails that linked the east and west coasts of the United States, thousands of workers endured desert heat, frigid mountains and countless obstacles. These rails in space will run in temperatures far hotter than any desert and far colder than any mountain," said NASA Mobile Transporter Subsystem Manager Tom Farrell at the Johnson Space Center in Houston. "And just like the transcontinental rails pulled together our country, these rails pull together 16 nations around the world, cooperating in orbit."

Atlantis will launch the railcar, called the Mobile Transporter, and an initial 43-foot section of track as it delivers the first segment of the International Space Station's exterior truss. Designated "S0 (S-zero)," the first section of truss will be carried aloft by Atlantis on shuttle mission STS-110 in April. More sections of track will be added during the next two years as eight segments of the girder-like truss are launched aboard the shuttle. By the end of this year, the tracks already will stretch more than 130 feet. When completed, the truss will stretch over 360 feet, the longest structure ever built in space.

An additional base system will be attached atop the flatcar- like Mobile Transporter during a shuttle flight in May, but the space train will leave the depot for its inaugural run during Atlantis' April mission. After spacewalkers loosen launch restraints and attach electrical and computer cable reels, Mission Control will command the Mobile Transporter railcar to inch its way up and down the 43-foot section of track.

"It's built for precise positioning and smooth velocity control; it's not built for speed," said Randy Straub, subcontract technical manager for the system with The Boeing Company in Huntington Beach, Calif.

The operation of the railway is critical for continued assembly of the station. It will allow the station's Canadarm2 robotic arm to carry future truss segments and solar arrays down the tracks to install them. Part flatcar and part locomotive, the Mobile Transporter weighs 1,950 pounds and is a horse made of aluminum, not iron. The Mobile Transporter was built by TRW Astro in Carpinteria, Calif., for Boeing, the prime contractor for station construction. It measures three feet high, nine feet long and eight feet wide and moves along two parallel rails attached to the station truss at speeds varying from one-tenth of an inch to one inch per second. Although driven by dual electric motors that generate only about a hundredth of one horsepower, the transporter can move 23 tons of cargo down the rails.

What is the hardest part about building a zero-gravity railroad?

"We've done a lot of work to make certain it can't jump the tracks," said Farrell. "We have to be sure it will be safe during all the station's activities, like reboosting its orbit or having visiting vehicles dock."

The transporter stays on track with three sets of wheels, one set that propels it and two sets in roller suspension units, spring-loaded units that have rollers on both sides of the track to ensure the transporter can't float loose. The railcar will have 10 stops, specific locations called worksites where it can be locked down with a 7,000-pound grip, allowing the robotic arm to safely maneuver cargo. Although it can be driven from the station or from the ground, the engineers for NASA's space railroad will normally reside in Mission Control, Houston, driving the train from thousands of miles away and hundreds of miles below.

Although the Mobile Transporter will be a freight train and not a passenger train, space- walking astronauts will have their own form of personal rail transportation aboard the station. Astronauts will operate a small handcar to maneuver up and down the rail line, a car that they will pull along the zero-gravity railway by hand to move themselves and their gear from place to place. Called the Crew and Equipment Translation Aid, two such carts will be delivered to the station before the end of the year.

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