For diesel locomotives pollution is a definite result. But for electric locomotive, it is dependant on the contributions for various sources feeding into the grid. Out of various sources for electric power the only polluting sources are coal and diesel/ Gas. so if a country is getting say 80% of its power from polluting sources. even then there is a reduction of 20% in pollution levels.
Indian Railways has taken a recent decision to go in for 100% electrification. That means that it will be going to be to be slow in induction of new diesel locomotives. The main reason behind this decision is to help clean up the air. On power generation side there is a simultaneous push for non-polluting technologies. So I see my country going in the right direction.