A new paper from Carnegie Mellon University didn't get much attention, but it tackles a huge topic: the challenge of cutting carbon emissions from the energy-hungry trucking sector.

One takeaway is that trains can play a big role, but that would require a reversal of freight industry trends worldwide that favor highways.

Why it matters: Moving freight around uses lots of oil and spews lots of greenhouse gases.

• Heavy trucking accounts for about a third of all carbon emissions from transportation, and road freight could account for 40% global oil demand growth over the next three decades, according to the International Energy Agency.
• "Rail intermodal transportation holds great potential for replacing carbon-intense and fast-growing road freight, but it is essential to have a targeted design of freight systems, particularly in developing countries," states the paper in Environmental Research Letters.

The big picture: Several strategies in concert are needed to cut carbon from heavy trucking — greater efficiency, electrification, lower-CO2 fuels, changes in supply chain management and more, the paper finds.

• "You need to push on all the levers," co-author Parth Vaishnav says.
• Lead author Lynn Kaack similarly emphasizes making trucking cleaner and moving freight off the roads. “You definitely need both for deeper decarbonization,” she tells Axios.

Where it stands: It's a tough problem, and here are a couple reasons why: current battery technology and economics aren't well-suited to long-haul routes, and very low-carbon liquid fuel replacements for diesel aren't yet mature either.

• “In the medium-term, the most promising strategy might be a shift from road to rail,” Vaishnav says.
• Right now, trucks handle about 60% of freight movement, and that's growing in most countries amid a "shift from rail to road," the study says.

What's next: It finds a "clear need for a systematic assessment" of the worldwide potential for shifting freight movement back towards rail, and the cost and emissions cuts associated with it.

The bottom line: "Cost-effective GHG emissions reductions for the transportation sector may be available but in today's markets will likely not lead to the levels of decarbonization that are needed to slow climate change," the paper states.

• "Thus, additional policies that include either incentives for reductions or penalties for GHG emissions will be needed."

Go deeper: Trucks are fueling the world's oil demand.