In the automotive world, diesel engines, here in the U.S. at least, are hard to come by. Part of this is because of the bad experiences Americans have had with them due to the early efforts of U.S. carmakers. Another reason we don't see too many here is because emissions standards for diesels are tough to meet without spending a good deal of money on after-treatment features. The manufacturers of the vehicles pass the money for the components that are required to make the engines legal to the consumer, and as I said, due to bad first experiences, many are reluctant to pay more for the efficiency benefits of the compression-ignition engines.
Despite the lack of choices in the automotive sector, boats and trains make widespread use of diesel engines. Trains have been using hybrid diesel/electric powertrains since the end of locomotive steam engines. The engines in trains must meet standards created in 1997, and I don't see any requirements for the marine diesel engines on the EPA site. A significant help to the emissions of these engines is being made with the low-sulfur diesel fuel that the EPA has mandated. But, the EPA has also proposed new emissions controls for the engines this month. For details on the changes and what impact they will have on emissions, click here for marine and here for locomotive.