Within minutes of venturing outside, John Herrington had discovered the obstruction: The railcar's cable-reel assembly was hung up on a radio antenna on the space station's newest girder.
"Houston, I found the problem," he called out.
Mission Control instructed Herrington to deploy the stowed antenna in hopes of removing the snag. At first, the antenna would not budge, but he finally freed it and cleared the path for the railcar.
"All right! Good job. Heroes both," astronaut Paul Lockhart radioed from inside the orbiting complex.
The spacewalk by Herrington and Michael Lopez-Alegria - their third in five days - had been planned all along so they could complete work on the newly installed $390 million girder.
Flight controllers initially suspected the railcar problem may have been caused by a momentary loss of computer data. But later, they said the railcar probably snagged on something while traveling down the tracks.
Astronauts inside the space station zoomed in on the area with cameras but didn't see anything unusual. So Mission Control asked the spacewalkers to check the railcar and two minicarts linked to it.
The $190 million railcar was empty when it abruptly stopped, after traveling about 45 feet, 10 feet short of its destination. It had just crossed onto the new girder, which was delivered by space shuttle Endeavour last week.
After a 5½-hour interruption, the railcar resumed its trip down the tracks and finally reached its intended location.
The space station's 58-foot arm was supposed to be maneuvered onto the railcar to serve as a crane during Saturday's spacewalk, but the plan was scrapped after all the difficulties.
Mission Control warned Herrington and Lopez-Alegria, visiting from Endeavour, that they would have to perform their work on the new girder without the use of the arm.
The astronauts had 33 clamps to install on joints in exterior air-conditioning lines to avoid pressure buildups. Lopez-Alegria immediately got started on the clamps, while Herrington, the first American Indian in space, was still struggling with the railcar.
This is the 49th spacewalk devoted to station assembly and maintenance and the 17th so far this year, reports CBS News Space Consultant William Harwood. Going into Saturday's excursion, 35 NASA astronauts, seven Russian cosmonauts, one Frenchman and one Canadian had logged 298 hours and 20 minutes building and maintaining the international space station, Harwood says.
The railcar will be necessary next year as a moving base for the robot arm so NASA can add more girders to the three already in place. This framework eventually will stretch 356 feet, from its current 134 feet, and support a network of solar wings and radiators.
Endeavour is supposed to undock from the space station on Monday and return to Earth on Wednesday with American astronaut Peggy Whitson and Russian cosmonauts Valery Korzun and Sergei Treschev, who spent six months on the orbiting outpost.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.