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Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains , since the application of the locomotive power to the train operates on each car in the train successively , and the power is thus utilized to start only one car at a time .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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