CAPE CANAVERAL, Fla. -- A wire service reports that astronauts sent the International Space Station's new rail car rolling slowly along its 44-foot track today, but it quickly hit a snag that interrupted the inaugural run.

NASA is blaming weightlessness. Engineers suspect that the one-ton car -- to be used during the next stages of the space station's construction -- floated slightly off the rail, causing its magnetic sensors to lose contact with a pair of iron strips in the aluminum tracks.

The problem cropped up after the empty flatcar had moved 17 feet, stopped at a work station and was trying to latch itself down. Everything came to a halt until ground controllers took over manual control.

A space station astronaut sent another computer command from inside to get the car moving again. It rolled smoothly for 26 feet to a second work station, where the same thing happened. Once again, ground controllers had to intervene.

"I think what we're finding out as we go through this is how the mobile transporter works in zero G," said Ben Sellari, a NASA manager.

The car is designed to transport the space station's 58-foot robot arm from one end of the outpost to the other. Its tracks sit along a 44-foot girder delivered to the space station last week by the shuttle Atlantis, whose astronauts plan a fourth and final spacewalk on Tuesday.