Six environmental groups will consider legal action if the Department of Transportation fails to act on a key rail safety rule, the groups wrote Thursday in a letter to Secretary Pete Buttigieg.

The rule in question would force railroads to begin upgrading freight trains’ Civil War–era braking systems to newer, electronically controlled brakes allowing for faster and safer stops. In 2017, after rail industry donors delivered more than $6 million to GOP campaigns, President Donald Trump’s administration repealed the 2015 rule requiring the newer brakes in some trains transporting hazardous materials, as the Lever reported last week.

In 2018, environmental groups, including Earthjustice and the Sierra Club, appealed the move by the Trump administration, citing a faulty cost-benefit analysis used to justify the repeal. An investigation earlier that year from the Associated Press revealed that the Trump administration omitted at least $117 million in estimated damages from train derailments when it determined that the costs of upgrading electronic braking systems would exceed the benefits.