A single railroad crosstie may not impact the environment as much as it helps to keep rails together. But considering that millions are deteriorating around the world, the material chosen as a replacement can affect the amount of carbon dioxide (CO2) in the air. Wood crossties require harvesting a lot of CO2-absorbing trees, roughly 89,000 cubic meters of timber per million crossties; concrete versions increase greenhouse gas emissions because of the fuel consumption during their manufacture. Robert H. Crawford of the University of Melbourne in Australia concludes that making enough concrete ties to keep one kilometer of tracks aligned for 100 years generates the equivalent of 656 to 1,312 metric tons of CO2. That amount is about one-half to one-sixth the amount that timber ties contribute, because concrete versions last longer and timber releases CO2 as it decays. Track the findings in the June 1 Environmental Science & Technology.