I have always been a fan of model trains. I don't know why, I just always have been. Lately, when studying efficiency and related things, I have become a fan of real trains. Though because trains have largely been unregulated as far as pollution goes, they are and have always have been a significant source of pollution, however, there are certain kinds that aren't, and the lack of regulation is being dealt with.
Trains have always been a method of moving moderately large volumes of things over great distances efficiently. Few people realize just how much stuff and how much mass is moving along in that train you see traveling through town. Each car can carry one to many times what a semi truck can carry, and at a better cargo to vehicle ratio. The only speed limits are those imposed by the construction and quality of the track, and those imposed by towns the tracks pass through. Efficiency is increased by limited grade that the train has to climb, and also the steel on steel wheels and tracks waste little energy to flex and heat.
As I said before, trains are now and have always been big polluters. The only evidence you need of that is to see the exhaust of a big modern diesel engine or to look at film of old steam engines chugging away at full steam. With pollution what you can't see is usually worse than what you can see, but what you can see is often indicative of what you can't. For instance, with coal fired steam locomotives, you can see smoke, but you can't see things like mercury, lead, fine coal dust, and NOx, stuff that are bad for you and the environment. Diesel engines eliminate the worst stuff but still leave some particulate matter, and are a significant source of NOx (nitrogen oxide, the biggest source of acid rain.) In fact, at this point, diesel engines account for up to 5% of the total NOx emissions in the country. Diesel locomotives are still vastly more efficient at moving stuff than cars though, remember that.
On the flip side, for many many years there have been electric locomotives. Electric locomotives share the same benefits and costs as electric cars except one important one. Electric locomotives need no batteries because they have overhead wires or third rails to provide a constant source of power. That means no fuel, no point of use pollution, more pulling power than other locomotives, and a really sweet one, because of regenerative braking, a train braking down one side of a mountain can provide most of the power for a train coming up the other side. This is put to great effect in mountainous areas of Europe where electric trains dominate. The downsides are as usual, cost. To electrify a rail system can cost as much as actually building the rails. As fuel prices rise, these economies will come into place and I am confident that electric rail will only expand.
Don't take an only dim view of diesels though, the EPA's Tier 2 locomotive requirements kicked into effect in 2005, requiring 65% less NOx, 50% less particulates and fewer other emissions as well. And unlike American car makers, GE's first run of the new locomotives easily came in under the emissions requirements and offer upgrade-ability for future requirements. The other benefits of emissions reduction as we have all seen are increases in efficiency which often pay for the changes in the first place. And biodiesel is a much better idea than ethanol because it can be made from wastes of all types and also algae.
The future: well, we seem to be ever slipping toward Europe, not that that's a bad thing, just saying. Electric trains are are probably the best mass transportation idea there is. I'd like to see them replace airplanes for the most part. If you crash a train, not everybody dies, I like that aspect. They often have sleeping quarters, better views, bigger windows, more spacious, and are cheaper. I like the idea of being able to hop a train and be in Oregon in less than a day, can't do it yet, but I like the idea. Also, it's kinda hard to crash a train into an important building.
I was recently at the Transportation Museum in St. Louis, here are some highlights.
World's Largest Tank Car, largest ever built, likely largest ever to be built.
Big Boy, one of nine or so left, most successful, kinda largest steam engine ever.
DDA40X, one of 13 left, longest most powerful American diesels ever. Almost 100 feet long.
Old street car.
Turbine powered engine. UP made some very large turbine engines way back that ran on propane or oil. I'd like to see one of those some day but there's only two left, in Chicago and Utah.
The museum also has on display one of three remaining operational Chrysler turbine cars.
Anyway, trains good, electric better.