When tweeting, this refers to a tweet that is either humorous or interesting that should be seen by the world. The RTtrain is a phrase added to a tweet asking for the tweet to be retweeted multiple times by your followers. The RT Train or RTtrain, is a good way to get a word or phrase to become a trending topic.
Person 1: Wow that last tweet was hilarious.
Person 2: You think so? Hop on the RT Train, let's get this baby trending!