The NGT order was passed following a PIL filed urging them to ask the Gujarat government to purchase only CNG vehicles for public transport bodies as diesel ones caused high level of air pollution.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/ahmedabad-transport-body-moves-gujarat-hc-against-ngt-order
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-ahmedabad
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/gujarat
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/national-green-trbunal-ngt
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/high-court
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/public-interest-litigation-pil
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/cng-buses
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution-control