The National Transportation Safety Board announced Friday that passenger cars involved in Tuesday's derailment will be inspected at an undisclosed location.
Air brake tests are planned to try to determine the cause of the incident.
"The reason we want to interview these employees is to compare their actual work activities with that of the policies and procedures of the railroad," said NTSB Official Mark Rosenker. "And then take a look at the policies and procedures of the railroad and make sure that they are up to the standards of the FRA."
The NTSB's investigation could take months to complete.