The metro system in the Indian city of Delhi has been praised for its positive impact on air quality in the area.
It is the first network in the world to be granted carbon credits by the United Nations (UN), as it continues to provide an environmentally-friendly alternative to cars.
Indeed, it is believed that up to 1.8 million people use the trams on a daily basis, many of whom would use their own vehicles if the public transport was not available.
As a reward for its positive effect on air monitoring results, the UN has allocated the system $9.5 million (£6.1 million) in carbon credits for the next seven years.
"No other metro in the world could get carbon credit, because of the very stringent requirement of the United Nations to provide conclusive documentary proof of reduction in emissions," a spokesperson for the metro commented.
Recent studies by scientists at the London School of Hygiene and Tropical Medicine highlighted definite links between exposure to air pollution and heart attacks.