“A dining car is a chew-chew train”

“Chew-chew train” (for “choo-choo train”) is an old joke. “This is presumably the dining car of a chew-chew train” was cited in the New York (NY) Times in 1916.
 
A different joke was cited in 1969:
   
Q: What kind of train carries bubble gum?
A: A chew-chew train.
   
   
17 September 1916, New York (NY) Times, “Little Stories from Here and There”:
During the rush hour, a middle-aged woman entered a subway car accompanied by her 18-year-old daughter. Both mother and daughter were forced to stand for a while opposite a row of men busily intent on chewing gum.
 
“Virginia,” said the matron with cutting emphasis and loud enough for the men to hear, “judging from appearances we have made a mistake. This is presumably the dining car of a chew-chew train.”
 
7 September 1969, San Diego (CA) Union, “Riddle Box,” pg. AA=7, col. 2:
What kind of train carries bubble gum?
A chew-chew train.
 
Google Books 
Forum: A Journal for the Teacher of English Outside the United States
1989
Pg. 57:
Daffy definition
Dining car: A chew-chew train
 
Google Books
Jest Another Good Clean Joke Book
By Bob Phillips
Eugene, OR: Harvest House Publishers
1996
Pg. 22:
Q: What do they call a train that carries bubble gum?
A: A chew-chew train.
 
Twitter
•̪ •
‏@practicecactus
Some trains have a dining car. These are called Chew Chew Trains.
7:33 PM - 26 Jan 2014
 
Twitter
Mackenzie Potter
‏@Mackinack
If the Amtrak dining car isn’t already called the “Chew-Chew Train” then this needs to happen. Like, right now. #seriousbusiness
7:36 AM - 27 Sep 2014