Astronauts to test drive space railroad

An Atlantis astronaut shot this photo of the international space station

(CNN) -- Space shuttle Atlantis astronauts should soon venture outside to hook up the first railroad in space, having completed the most difficult task of their 11-day mission, attaching a $600 million beam to the international space station.

The 13.5-ton girder will serve as the backbone for future research and construction projects on the exterior of the space station, as well as house a depot for a mini-railroad on the orbiting complex.

The Atlantis crew, which moved the massive truss to the space station on Thursday, will conduct their second and third space walks over the weekend to complete installation of the structure, a squat, metal, three-dimensional trapezoid brimming with electrical wires, cables and computers.

Also during the Sunday trip outside, the spacewalkers plan to place a $190 million railcar to a railroad segment attached to the truss.

•  Shuttle mission guide  •  CNN Presents: what makes the shuttle fly Movies:  •  Atlantis returns to earth  •  Atlantis leaves space station  •  CNN's Miles O'Brien talks with shuttle crew  •  CNN's John Zarrella previews the mission  •  Watch the launch from Kennedy Space Center  •   NASA animation of completed space station railroad More resources:  •  Cult 3-D model of the space shuttle  •  Milestones in space shuttle history  •  Space: The final frontier for railroads

With the addition of new segments in the coming years, the railroad should stretch the length of a football field, allowing the station's robotic crane to move from one end of the sprawling modular complex to the other.

The new railcar should move for the first time on Monday. A test is planned to move it up and down the first length of track, which extends about 44 feet (13 meters).

After a weeklong rendezvous, Atlantis should undock from the station on April 17. Shuttle astronauts expect to return to the Kennedy Space Center in Florida two days later.

Besides nearly $1 billion in heavy hardware, the shuttle carried along sensitive scientific experiments. One known as the Photosynthetic Experiment and System Testing and Operation, or PESTO, could lead to better space food.

The wheat growth project, which will remain on the station at least two months, could help scientists determine how to grow grains in space, raising the prospect of astronauts baking their own bread.

The shuttle astronauts are the first visitors to the space station since its current crew of two Americans and one Russian arrived in December. The station residents expect to return home this summer.