We've enjoyed every minute with them, and it's going to be hard to leave them here

Michael Bloomfield, Atlantis commander

During their visit, they installed a 44-foot-long (13 metres) girder that will act as the backbone for the ISS as it continues to grow over the coming years.

Spacewalkers from the Atlantis vehicle attached a one-tonne railcar to the girder to help future workers shift around their equipment and building components more easily.

Fond farewell

The final walk on Tuesday saw the astronauts finish those small tasks needed to complete the installation of the girder.

In a farewell ceremony on Wednesday, shuttle commander Michael Bloomfield said the three-strong crew on the ISS had made the visiting astronauts feel at home.

"We've enjoyed every minute with them, and it's going to be hard to leave them here," he said.

The station's Russian commander, Yuri Onufrienko, added: "A very big thanks, everybody. That was really a great job, and we'll see you soon on the ground."

American space agency (Nasa) engineers back on Earth will continue to assess the glitches that have hit "the first railroad in space".

Every time the railcar moves along the girder to a workstation and comes to a halt, its automatic latching system shuts down - ground controllers have to secure the car via a manual computer command.

They suspect the car floats ever so slightly off the rail, causing the magnetic sensors on the bottom of the car to lose contact with a pair of iron strips in the girder's aluminium tracks.

Evolving platform

Nasa stresses there is no danger of the car ever floating off into space because safety rings secure the trolley to the platform.

Ben Sellari, a Nasa manager, said engineers might have to adjust the computer software if the problem became more of a nuisance.

The railcar will be needed later this year to transport the space station's 58-foot-long (17 metres) robot arm from one end of the platform to the other, as more girders are installed.

The Atlantis orbiter is due back on Earth on Friday 19 April.

The space station's current three-member crew still has two months left in its six-month tour of duty.