Trains hauling oil and other flammable material should be better and more frequently inspected, a National Academies of Sciences report out Wednesday says.

Why it matters less today: The amount of oil shipped by rail has dropped 77% since its high in 2015.

Expand chart Data: U.S. Energy Information Administration; Chart: Lazaro Gamio / Axios

What's driving the drop: "Trains receded into the background after the oil price fell and pipelines started up, both of which took pressure off the rail network," said Kevin Book, managing director of ClearView Energy Partners, an independent research firm.

The backstory: A series of fiery oil-train crashes over the last few years prompted federal regulators to issue a rule, finalized in May 2015, strengthening requirements for trains hauling flammable material like oil and ethanol. President Trump has targeted most Obama administration regulations for repeal, but so far he hasn't said anything about eliminating or scaling back this one.

Pipelines > rail: While rail offers companies more flexibility on where to move oil, pipelines are cheaper and safer, so they're generally preferred as the default oil mover where and when possible, most experts agree. If there's another big oil boom, you can expect oil-by-rail shipments to go back up, and that's where this report's findings and the regulation would come back to the front burner.

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