This paper presents a methodology to measure the environmental co-benefits of transport initiatives, defined here as carbon emissions in conjunction with local air pollution. An evaluation tool was developed and then tested on the case of the Delhi metro. The metro is an extensive rail project spanning the Indian capital, which is also the world's first rail based Clean Development Mechanism (CDM) project. However, it has also been a flashpoint in urban policy-making in the city in recent years. This analysis identifies the co-benefits based on the current situation as well potential co-benefits based on increased ridership and altering mode share contributions. The paper then discusses the challenges faced in the quantification process and the practical implications of achieving increased co-benefits. The paper highlights issues of data quality as well as data access, whilst identifying that factors peripheral to the project itself can have a critical effect in achieving co-benefits from large scale transport infrastructure projects in developing countries. (C) 2013 Elsevier Ltd. All rights reserved.