If you are a parent or grandparent, then you know exactly what this Question of the Day is about. If you are single or married-without-children, you may have never heard of Thomas the Tank Engine and his friends. Thomas is a merchandising phenomenon that includes small wooden train sets, video tapes, books, clothing, and even a web site (http://www.thomasthetankengine.com). I have a 2-year-old son who loves trains, so I understand completely…

Take a look at this page: http://www.puffins.com/lcTTpg1.html. It has a nice set of pictures of Thomas and the other engines in the series. There are two things to notice in these pictures:

  1. Edward, Henry, Gordon and James all have coal cars.
  2. Thomas has no coal car, and there are also odd square tanks beside his cylindrical boiler. The other engines do not have these tanks. If you look carefully at Thomas, you will notice that he's also carrying his own coal in a small bin behind the cab.
The square tanks beside the boiler on Thomas are what make Thomas a "tank engine". In real life the tanks might hold between 1,000 and 2,000 gallons (4,000 to 8,000 liters) of water, depending on the engine. On the other engines in the series, the coal car shown is simplified. In a real steam engine, the coal car carries both coal and thousands of gallons (up to 20,000 gallons or 80,000 liters on the biggest engines) of water (see This photograph of an authentic steam engine that runs in Colorado - coal is up front, and water is in back of the coal car). This water is needed because most steam engines vent their used steam through the smoke stack rather than condensing and reusing it. All engines except tank engines need to carry these coal/water cars right behind the engine.

Tank engines evolved as a way to handle short lines and switching duties in a train yard. The engine carried a small amount of coal behind the cab and perhaps 1,500 gallons of water in its tanks. A tank engine is therefore self-contained and does not need the coal/water car. This makes it lighter, smaller and less expensive, but gives it a limited range before it must be re-coaled and re-watered. Tank engines were not very common in the U.S. but were very common in England.

So Thomas the Tank Engine is a fairly authentic depiction of a form of short-haul steam engine! The other engines in the series are "normal" steam engines pulling separate coal/water cars.