Bella Dinh-Zarr, of the National Transportation Safety Board (NTSB), told reporters that positive train control (PTC) – a safety system that automatically slows or stops trains that are going too fast – was not operational on the train.
“The locomotive was in the process of getting a system of PTC installed but it was not yet functional,” she said.
She added that the board had long recommended that PTC be installed across all US rail systems.
But, she added, Congress had extended the deadline for implementation of the legislation from the end of 2015 until the end of 2018, a delay she described as “unfortunate”.
The cost of implementing the system fully on all tracks and vehicles is reported to be more than $22bn (£16.4bn).
More>>