BILLINGS, Mont. — Accident-prone tank cars used to haul crude oil and ethanol by rail could remain in service for another 15 years under federal rules that allow companies to phase in upgrades to the aging fleet, according to the U.S. National Transportation Safety Board.

Transportation officials and railroad representatives have touted the rules as a key piece of their efforts to stave off future disasters, following a string of fiery derailments and major spills that raised concerns about the crude-by-rail industry.

Yet without mandatory, periodic benchmarks for meeting the requirements, the decision to upgrade to safer tank car designs “is left entirely to tank car fleet owners, and may be driven by market factor influences, not safety improvements,” NTSB Chairman Christopher Hart said in a letter Tuesday to the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration.

Tom Simpson with the Railway Supply Institute, which represents tank car manufacturers and owners, said the industry is committed to putting stronger cars in place. Members of the group will meet deadlines for replacing or upgrading the cars, he said, noting that demand for rail cars has eased after crude-by-rail shipments decreased over the past two years in response to lower oil prices.