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The long rigid wheelbase of the conventional steam locomotive becomes a problem on tightly curved rail lines, or when the size of the locomotive increases. While leading and trailing wheels are able to move from side to side to accomodate curvature, the movement of driving wheels is restricted. The problem can be alleviated a little by using lateral motion devices on some or all of the driving axles, permitting a small amount of sideplay, and by having blind (flangeless) wheels on some of the driving axles.

These can only be taken so far, however. Some form of articulation was the obvious solution; providing some means or other by which multiple sets of driving wheels could move independently, generally by hinging the locomotive frame in some manner.

Many kinds of articulated locomotives were devised. Fairlies, Mallets, Garratts, and Kitson-Meyers were all designs of articulated locomotive. Geared locomotives like Shays, Climaxes and Heislers were articulated locomotives too.

A number of modern high speed trains such as the TGV and the Eurostar are articulated rather than coupled.

In conventional trains, each carriage has a bogie at each end which swivels on a central mount, providing the ability to take corners. And each carriage is then joined together with a coupling that provides a physical linkage as well as power, signalling, control etc.

Many high speed trains take a different approach. Rather than a bogie at each end of every carriage, there is a single "truck" at each join, which supports the ends of two adjacent carriages. This means the entire train is articulated together as a single unit. (Eurostar trains actually are coupled together in the middle - this is so if there's a problem in the Channel Tunnel, they can move all the passengers to one half of the train and drive it out leaving the failed half behind. metalangel also reminded me that there is a conventional coupling between the actual locomotives at either end and the carriages themselves. Often on the modern trains (such as the Eurostar), these are high-tech automatic couplers that make the physical link and all the electrical links without human intervention.

The biggest single advantage with the articulated linkage is rigidity. As a result of derailment, traditional train carriages can jacknife around, crashing into each other and whatever else is beside the track. This of course can cause serious injury and damage. With the articulated design, the train stays together and travelling in the same direction, slowing but not moving laterally. This usually results in far less injury and damage. On the other hand, the ICE (the German high speed train which uses conventional coupling) had a high speed accident and did jacknife causing injury to the passengers.

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