Updating the design of the tank cars — DOT-111s, dating from the 1960s — has been a far more vexing question for regulators in the United States despite warnings by safety officials for more than 20 years that those tank cars were prone to rupture in a derailment.

New standards have languished in a slow rule-making process since 2011, when railroads requested that regulators toughen tank car standards. The National Transportation Safety Board, in 2009, said defects on these cars contributed to the explosion of an ethanol train that killed one person in Cherry Valley, Ill. The Pipeline and Hazardous Materials Safety Administration formally initiated the process in September 2013, but has been under pressure to release its new rules quickly after a string of accidents and derailments involving oil trains.

Image Anthony Foxx, the transportation secretary, said Thursday that tank cars needed to be retrofitted with better protections. Credit... Erik S. Lesser/European Pressphoto Agency

Visiting Casselton, N.D., where an oil train exploded in December, Transportation Secretary Anthony Foxx said on Thursday that tank cars needed to be retrofitted with better protections or replaced, but he provided no details about the proposed new standards. “Our rule-making process requires that we go through several steps, and that’s what we’re going through,” he said.

The rules will be published in the Federal Register once they have been reviewed and will then be opened for public comments for a period of 60 days.

The issue is particularly vexing because it pits three powerful industries — oil companies, railroads and shipping companies — whose interests do not always align even as they do business together, against each other. All have at one point or another sought to deflect blame for the accidents and explosions. By law, railroads cannot turn down any product from being shipped on their rails.

Also, while many trains carrying oil have derailed in the United States, they have generally done so far from populated areas, though the derailment near Casselton happened right outside town and prompted the voluntary evacuation of hundreds of residents.