The freight train derailment and explosion that flattened a tiny Quebec town earlier this month has renewed scrutiny of a widely used type of oil tanker cars that has been a concern by safety watchdogs for years.

The tanker cars involved in the crash — known as “DOT 111s” because they are designed to meet the standards required by Transportation Department Specification 111 — account for almost 70 percent of the railcars that carry oil in the United States, according to industry statistics. The National Transportation Safety Board has cited concerns about designs of the tanker cars after previous accidents and the Pipeline and Hazardous Materials Safety Administration is currently developing new regulations covering the transport of hazardous materials by rail, including standards for the DOT 111 tanker cars.

The accident in Lac-Mégantic, which is believed to have killed more than four dozen people, has drawn attention from officials in Maine because the train had been scheduled to traverse northern Maine en route to a refinery in New Brunswick.

Noting that oil shipments across Maine by rail have increased 15 times — to 30,000 barrels a day from 2,000 — the state’s two Democratic House members wrote last week to the top officials at the NTSB, Federal Railroad Administration and PHMSA.

Reps. Michael H. Michaud and Chellie Pingree asked the officials to identify “any weaknesses in existing infrastructure, and describing best practices to address any deficiencies you find.”

Two Democratic aides closely watching the investigation say the response by regulators is likely to focus largely on the DOT 111 tankers. It is possible, they said, that the outcome could prompt action by Congress as part of a rail reauthorization bill that lawmakers expect to write this year. As it was in the 2008 authorization (PL 110-432), safety is expected to be a central focus of lawmakers writing the next rail bill.

The design of the tanker cars has garnered criticism by safety officials for more than two decades.

After a 2009 derailment in Cherry Valley, Ill., of a train carrying ethanol, the Association of American Railroads developed requirements designed to improve safety on newly manufactured DOT 111 cars, including thicker shells and head shields to better withstand unintended impacts. The new standard took effect on Oct. 1, 2011, but safety regulators say the safer cars would be phased into use too slowly.

“The revised [Association of American Railroads] standard would address tank cars constructed after the changes are published and would not be expected to require retrofitting of the tank car fleet existing at the time the changes are published,” NTSB Chairwoman Deborah A.P. Hersman said in a 2012 letter to Pipeline and Hazardous Materials Safety Administration chief Cynthia L. Quarterman. “Given the estimated tank care service life of 30 to 40 years, this represents the potential for tank cars with susceptibility to tank failure from loads applied to the draft sill to exist long after changes are made to the design standards.”

In its June regulatory update report, the Transportation Department said the pipeline regulatory agency intended to publish a proposed rule this month with a public comment period running through mid-September. So far, the agency has not submitted a rule to the Office of Management and Budget for review.

A spokesman for the regulatory agency said the current rules describe the type of rail cars permitted to carry materials like crude oil and set requirements for construction and maintenance.

The revised regulations “would further clarify the regulations,” the spokesman said.

The timing of the Lac-Mégantic accident is likely to elevate the issue of tanker car safety as the House Transportation and Infrastructure Committee and Senate Commerce, Science and Transportation Committee write the next rail bill.

With the current authorization set to expire at the end of September, freight railroads were already pressing to delay a congressional mandate for installation of automatic anti-collision systems by 2015. The Quebec accident may complicate the industry’s campaign for a reprieve from the safety mandate.