Trump certainly gutted or tried to gut many safety and/or environmental requirements, often with completely absurd justification [1], but I'm not sure this particular case is such.

The safety requirements in question, mandating ECP brakes on certain trains carrying toxic cargo (I think it was trains above a certain length unless they stayed below some specified speed or something like that), were enacted under the FAST Act in 2015.

One of the requirements of that was that before their scheduled effectiveness date that the National Academy of Sciences was to investigate this and determine if the assumptions made when requiring ECP brakes were justified.

My Googling has failed to turn up a copy of the NSA report, but from the small amount that has been quoted in newspaper and magazine articles that did turn up, and from what DOT said when they announced in late 2017 that the ECP mandate would be rescinded, it sounds like the NSA was unable to come to a conclusion on the relative performance of ECP brakes compared to other brakes in emergencies.

Here's what DOT said:

> In accordance with the Fixing America’s Surface Transportation (FAST) Act (Public Law 114-94), the U.S. Department of Transportation today announced the final updated Regulatory Impact Analysis (RIA) in regard to Electronically Controlled Pneumatic (ECP) brakes on certain trains. After careful review in accordance with the Congressional mandate contained in the FAST Act, the Pipeline and Hazardous Materials Safety Administration (PHMSA) and the Federal Railroad Administration (FRA) will rescind the ECP mandate.

> This determination was made with congressionally-mandated input from the National Academy of Sciences’ Transportation Research Board, U.S. General Accountability Office (GAO) and studies by the FRA, which found that the cost-benefit analyses are not sufficient justification for mandating ECP brakes.

> The National Academy of Sciences determined it was unable to make a conclusive statement regarding the emergency performance of ECP brakes relative to other braking systems. In addition, the updated RIA incorporated recommendations from audits conducted by the U.S. General Accountability Office and updated costs and benefits of the ECP brake provision based on current economic conditions. This review demonstrated that the costs of this mandate would exceed three-fold the benefits it would produce.

If the NAS really found that there wasn't enough evidence to say ECP brakes were actually better and the GAO really found that the costs exceeded benefits by 3 to 1 and they were using whatever value is normally used on human life when doing cost/benefit analysis...then this sounds like it might have been destined for repeal regardless of who was President at the time.

I'm curious about why the FAST Act required such a review in the first place. Is that normal? I'd kind of expect such review to come before, not after.

[1] E.g., their weakening of the Migratory Bird Treaty Act (MBTA). The MBTA implements a treaty from the early 1900s between Canada, the US, and Mexico to protect birds that migrate between them. So for example suppose I have some land that migratory birds covered by that treaty rely on as a nesting ground, and want to say build a parking lot on that land. I'd have to do so in a way that does not harm the birds. At least that's what everyone thought for the prior 100 years, including nearly everyone involved in making or ratifying the treaty.

Trump's administration decided we've all misunderstood what it meant all those years. What it actually meant is that you could not intentionally harm the birds. So if I wanted to pave over my land to make a parking lot because I wanted to kill the birds, that would not be allowed. But if I wanted to pave over my land to make a parking lot because it would make money and killing the birds is just an unintentional side effect, that would not be covered by the treaty.