NASA's one-year spaceman, Scott Kelly, and astronaut Timothy Kopra exited the space station's hatch early Monday ahead of schedule.
The spacewalk was unplanned.
NASA managers want them to move the rail car so a cargo ship filled with almost three tons of food and supplies can dock on Wednesday.
Engineers believe a stuck brake handle is to blame. The rail car needs to move about four inches (10 centimeters) so it can be latched into place.