Researchers use regenerative braking to put direct air capture onto railway tracks

This is the claim of CO2Rail Company, a Texas start-up that has worked with several researchers, including engineers from Sheffield University, to design Direct Air Capture (DAC) technology that can be used within special rail cars placed on trains in regular service.

The DAC rail cars work by using large intakes of air that extend up into the slipstream of the moving train to move ambient air into a large cylindrical CO2 collection chamber, eliminating the need for energy-intensive fan systems currently necessary with stationary DAC operations. 

The air then moves through a chemical process that separates the CO2 from the air and the carbon dioxide free air then travels out of the back or underside of the car and returns to the atmosphere.

After enough has been captured, the chamber is closed and the harvested CO2 is collected, concentrated, and stored in a liquid reservoir until it can be emptied from the train. It can then be used as value-added feedstock for CO2 utilisation, or sequestered into suitable geological sites nearby.