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This free movement results from the fact that in railroad practice cars are loosely coupled , and the coupling is often combined with a shock-absorbing device , a " draft gear , " which , under stress , substantially increases the free movement as the train is started or stopped .
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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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