"X Train" is a train that Grouches use when they go on a vacation. A Sesame Street song sung by Oscar & Grundgetta on Sesame Street mentions the same train. The 2 Grouches are shown at Oscar's trash can together. Oscar is holding a small picture of the letter X & Grundgetta is holding a small picture of a train.
They start to say "X" & "train" to each other over & over, something Gordon only sees as an argument, & so he asks them what it is they're arguing about this time. Immediately, the Grouches insist they're not arguing, just getting ready to sing a sing about a train, which is called the "X Train", which is named cause it's a train that has a big letter X on it.
Oscar & Grundgetta then start singin a song about how Grouches travel on the X Train, a train that, apparently, doesn't take any of its passengers anywhere, & that the train whistle is the only thing that they can ever get going, which Grouches obviously never mind, being who they are.
Then, after the song, Oscar asks Gordon if he has any questions about the X Train, to which Gordon replies "yes.", but Oscar says "Too bad! I'm not answering them!", & obviously Grundgetta says "Ha! Me either!", & they start laughing as they (presumably) head back down into Oscar's trash can.
- Sesame Street (Episode unknown)