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1.
A series of people yawning consecutively.
Man, Mr. Nguyen's math class really started a yawn train!
by goingoncegoingtwice September 28, 2010
 
2.
When someone yawns and then another person yawns and then everyone yawns and nobody notices it.
"I bet if I yawn I can start a yawntrain."
by Yawntrain May 09, 2012