(CNN) -- Federal accident investigators issued two "urgent" recommendations to the Chicago Transit Authority on Friday urging action to prevent another crash similar to one this week in which an unoccupied CTA train collided with another at a station, injuring 33 people.
The National Transportation Safety Board told the authority in a letter that it needed to take steps, including using wheel chocks, to prevent trains from moving unintentionally.
CTA said in response it had taken those and other steps immediately after the crash on Monday.
The accident occurred when a four-car train -- unoccupied but powered up -- departed the Forest Park Repair Terminal where it was awaiting repairs.
It traveled almost a mile downhill through five devices designed to stop trains, eventually colliding with an occupied train stopped at the Harlem Station on CTA's Blue Line.
Some 33 passengers were injured. Those who were hospitalized have been released, authorities said.
The empty train movement remains under investigation, but the CTA said neither criminal conduct nor vandalism is suspected.
The safety board said unoccupied CTA trains are "routinely left powered up" while stored at the repair yard. But CTA spokesman Brian Steele disputed that, saying trains are typically powered up prior to going into service.
Steele said preliminary indications show the train was powered up at the time of the incident, but it was unclear when it was scheduled to depart the repair yard to go to another maintenance yard.
"That's one of the questions we are trying to answer," he said.
The NTSB said the investigation continues, noting that one of the cars had heat-damaged wiring and water in electrical connection boxes.
Steele said the CTA has inspected about half of the 600 to 700 cars in its fleet without finding similar damage. The inspections were to be completed this weekend, he said.
The NTSB typically rolls out recommendations at the conclusion of investigations, but can issue guidance earlier if it identifies a safety problem.
In this case, it issued the recommendations while most of the agency was on furlough because of the government shutdown.
The furlough rules allow for ongoing investigations in cases that are necessary to "prevent the imminent potential for loss of life and significant property damage." The NTSB also can launch investigations into "major accidents involving significant casualties."
The actions recommended by the board need "to be addressed expeditiously to prevent a recurrence," NTSB chairwoman Deborah Hersman wrote in the letter to Forrest Claypool, president of the CTA.
In addition to wheel chocks, the NTSB recommended turning off propulsion power and using an alternative brake setting, and a system to derail cars before they enter the main track.
The NTSB also wrote to the Federal Transit Administration requesting it issue a safety advisory to all transit systems asking them to review operating and maintenance procedures for stored, unoccupied cars. All but 28 of the FTA's 529 employees have been furloughed.
An FTA spokeswoman said that the administration complied and issued the safety advisory.