(CNN) -- The first ever U.S. National Train Day on May 10 was a celebration that may have passed many Americans by. But why create a day to celebrate trains?
Shanghai's maglev train: White elephant or great green hope?
According to Amtrak, which was behind the event, trains are more energy-efficient than cars or planes so should be celebrated and actively encouraged as the ideal mode of transport among today's travelers.
This isn't the first time that the train has been pitched as one potential answer to the world's transport-related environmental woes.
In June last year, Greenpeace went as far as offering free train tickets to British Airways (BA) passengers in protest at the increasing number of short-haul flights the airline was offering out of the UK.
And when Germany declared it wanted to become the most energy-efficient country in the world by 2020, one of the ways it said it was planning to do this was by encouraging its citizens to ditch the plane and take the train.
The Nature Conservancy, which calls flying "the opposite of green" on its web site, says by opting for train journeys your passenger mile emissions could be anywhere from 4 percent to 15 percent of those you would generate by flying the same distance, depending on the length of the trip and the efficiency of the train service.
According to Eurostar, by taking one of their cross-Channel trains between London and Paris or Brussels, you would be emitting 10 times less carbon dioxide than if you flew.
"Definitely, if used as a new basis of transport, trains use less energy and are more efficient at reducing pollution," says Professor Shen Jianfa, who works in the Department of Geography and Resource Management at the Chinese University of Hong Kong (CUHK).
"But you have two things to consider -- the cost of transport and its efficiency if compared to bus or truck. For long distances rail is more efficient."
Some not buying the green ticket
But some warn that taking the train does not always equate to a completely green option.
The Cato Institute recently argued that the majority of America's rail transit lines as a whole are less energy-efficient on a passenger mile basis than you might think.
In an April 2008 report Cato said the U.S.'s train lines "generate more greenhouse gases than the average passenger automobile," before adding,"rail transit provides no guarantee that a city will save energy or meet greenhouse gas targets."
It cites one main problem as "feeder" bus operations that are set up to carry passengers between train stations or on to their final destinations. When they are operating at low capacity, these buses can counteract any environmental gains you may have made with train travel, Cato says.
But CUHK's Shen argues that having a half empty feeder bus has to be preferable to all those passengers using their cars instead.
"You have to provide a shuttle bus to get people home, but this depends on the population density of the city," he says.
"If people are living in a large area with low density then a railway line can't reach all the people. There has to be a point where it is more efficient to have feeder buses."
Cato also cites the issues of constructing and maintaining rail lines, which it says would take "many decades of energy savings" to recover the original energy costs involved.
And then there is the issue of what fuel you use to power the train in the first place.
The U.S. Environmental Protection Agency (EPA) recently took trains and ships to task by imposing stringent new emissions standards that will now force up to 21,000 diesel-powered trains to slash their soot emissions by 90 percent and nitrogen oxide emission by 80 percent.
According to the EPA web site, diesel-powered trains "are significant contributors to air pollution in many of [the country's] cities and ports."
In Asia, however, the train systems tend to be powered by electricity, making them by and large a cleaner option than equivalent train journeys in the U.S.
That being said, where that electricity is coming from is still a cause for concern, Shen says.
"Along the line [in China] it is electric but obviously you need to know where the electricity is coming from," he says.
"For China as a whole they rely very much on coal."
All aboard - if the price is right
Keeping train fares low is key to making any train service a success and government subsidies are key here, argues Christine Loh, chief executive of Hong Kong independent think tank, Civic Exchange.
"The government should pay for the construction cost of the rail line so that fares will be kept low," she says, referring to Hong Kong's need for a more extensive rail system.
Because trains, particularly clean ones, don't always come cheap.
Japan recently laid claim to the world's first diesel hybrid train, but its creators, East Japan Railway told the AP news service that the Kiha 200 train cost $1.7 million to build -- twice as expensive as their standard trains.
And that's small change when you look at the cost of Shanghai's first ever maglev train - a train that literally glides over the track without making contact, powered by magnetic levitation technology.
As maglev trains cannot travel along traditional rail tracks, new ones have to build - and in the case of the 30 kilometer line between Shanghai's Pudong Airport and the city center - at a cost of $1.2 billion, according to China Daily.
Cost is the main reason Shen believes more subway or overground lines have been slow to appear in Hong Kong, a small high density city with one of the lowest car ownership rates in the world - but which still suffers from high levels of pollution.
"The government has a strategy on paper to build more rail lines but building more MTR lines is very expensive compared to highways," Shen points out.
But train lines are becoming a necessity for growing Asian cities that are becoming more crowded with more traffic congestion and pollution.
"In places like Beijing and Shanghai, we are seeing an increase in private cars but over the last few years the city governments have been realizing that they can't rely on cars for transport because of the congestion," Shen says.
"China is being very ambitious in expanding the rail network. In the next 10 years the rail network will be doubled," he adds, citing the government's plans to build an express rail between Beijing and Shanghai in addition to one between Hong Kong and Guangzhou.
"Efficient, cheap and extensive rail systems are attractive to large cities," adds Loh.
And if you want to see an example of that in action, she says look no further than Tokyo.
"This is a large metropolis that is densely populated. The government's choice was to build an extensive subway/rail system that is cheap and efficient to ensure it is the backbone of the public transportation system," Loh explains.
"Buses are complementary to the rail system, and taxis are extremely expensive. Driving to work is prohibitive for most workers. Japan also makes sure old vehicle models are retired periodically so there are no highly polluting old vehicles on the roads.
"This package of measures work," says Loh.