Shuttle astronauts have a fourth and final spacewalk to complete on their current construction mission to the International Space Station (ISS).

They have already fixed to the orbiting platform 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 have even attached a one-tonne railcar to the girder to help future workers shift around their equipment and building components more easily.

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

Software change

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.

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

Coming home

Ben Sellari, a Nasa manager, said engineers might have to adjust the computer software if the problem being experienced 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 to detach itself from the ISS on Wednesday for the trip home to Earth, landing on Friday, 19 April.

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