What is a tank engine, as in Thomas the Tank Engine?

If you are a parent or grandparent, then you probably know exactly what this Question of the Day is about. Thomas the Tank Engine is a merchandising phenomenon that includes small wooden train sets, video tapes, books, clothing, an official Web site and tons of fan sites. I have a son who loves trains, so I understand completely.

This page has a nice set of pictures of Thomas and the other engines in the series. There are a few things to notice in these pictures:

The square tanks beside the boiler 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 of water (up to 20,000 gallons or 80,000 liters on the biggest engines). 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 (5,700 liters) 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 pretty limited range before it must be re-coaled and re-watered. Tank engines were not very common in the United States, 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.