Until recently, exhaust emissions from the marine and rail sectors were often overlooked. However as other sources of pollution have gradually been reduced, attention has turned to emissions from these sources, especially in or near cities. The often large size of the engines involved is one important factor behind this shift, with another being the relatively old-fashioned technology employed, due both to the durability of the engines themselves and the lack of regulation. The increased availability of low sulphur fuels has made the use of exhaust gas aftertreatment systems feasible, and these can now be installed on ships, boats, locomotives and railcars.
Since regulations are generally related to the location where they are operated, marine and rail engines tend not to have any exhaust-gas aftertreatment systems fitted as standard. This means that they are prime markets for retro-fitting, and major reductions in the emissions of toxic by-products of combustion can be achieved.