The Ringling Bros. and Barnum & Bailey circus performs often at Madison Square Garden in New York City, but the circus is also frequently on the road. The performers are fed in a railroad dining car that’s known as the “pie car.” The pie car serves more than pies—the dessert that circus clowns often throw at each other.
 
The term “pie car” appears to date to at least the 1930s.
   
   
Circus Terms
Cookhouse
Place where circus people eat. Also Pie Car.
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Pie Car
Place where circus people eat. Also cookhouse.
   
Circus Trains
Pie Car - The Pie Car is the closest thing the circus train has to a railroad diner or club car.  The Pie Car served as the social gathering place while the train was in route.  Sometimes, the operation was let out as a concession and on other shows it was run the by the circus management.  The Pie Car offered short-order food and some had a bar.  Circus personnel could usually find a game of cards or dice in progress on the Pie Car.  Normally, the Pie Car did not serve meals for the show people.  One exception was the Nickel Plate Shows which had no cookhouse and fed its personnel three meals per day in the Pie Car.  The Pie Cars on today’s Red and Blue Units of the Ringling Bros. circus are dining cars and a large portion of those shows’ personnel eat their meals in the Pie Car.  However, these modern day Pie Cars still offer short orders from an early breakfast to a midnight snack.
 
Ringling Brothers and Barnum & Bailey Circus
Pie cars
This car provides meals for the circus members. By tradition the diner car is called the “pie car.”
 
Google Books
“Hey Rube”
By Bert J. Chipman
Published by B.J. Chipman
1933
Pg. 30:
Circus followers crave their pie, ala mode when possible. (...) [R]emarking, “that’s the first time I ever got what I ordered in this pie car.”
   
24 July 1954, Williamsport (PA) Gazette and Bulletin, “Lycoming County Fair to Have ‘Biggest, Brightest’ Midway,” pg. 5, col. 2:
The show train also carries on stock car, the “pie car” or diner, and four sleepers plus Mr. Strate’s private car.
   
24 May 1964, Los Angeles (CA) Times, “Pie Car” by Clementine Paddleford, pg. A16:
The nomadic Hotel Ringling has the largest traveling restaurant in the world. This 90 foot-long diner is one of the Silver Arrow Fleet of 19 railroad cars which carry “The Greatest Show on Earth.” The circus train averages 20,000 miles a season into 20 states.
   
9 July 1964, Dallas (TX) Morning News, “Circus ‘Pie Car’ Cooks for 1,400 Daily,” section 1, pg. 17:
Even circus performers have to eat. And the Ringling Bros. and Barnum & Bailey restaurant, attached to the Ringling Silver Arrow Fleet of 19 railroad cars, feeds about 1,400 mouths daily.
 
“Pie Car,” as it is affectionately called, is the largest traveling restaurant in the world, averaging 20,000 miles a season. Food is served around the clock except for one hour between two and three in the morning when the cleanup crew moves in.
 
Until 1957, the Ringling Bros. dining room was set up under canvas at various stops. But now, since the circus no longer plays under tents, the silver Pie Car is everybody’s home. Only star actors have their own compartments equipped with kitchenettes.
 
28 March 1969, Washington (DC) Post, “Circus Talk” by Joan Kramer, pg. D8:
...says she never uses “nickys” (nicknames).  Frankie Saluto, the 39-year veteran midget-clown, uses them though. The ones he knows best are “pie car,” the area where he eats dinner of the train, and “ghost walk,” which is how he fondly refers to pay day.
   
Google Books
The language of American popular entertainment: a glossary of argot, slang, and terminology
By Don B. Wilmeth
Published by Greenwood Press
1981
Pg. 200:
Pie car: Circus and carnival term for the dining car of a railroad train.
 
Austin (TX) American-Statesman
Pie car on circus train brings everyone together
Food on the road is a production in itself
By Addie Broyles
AMERICAN-STATESMAN STAFF
Saturday, August 23, 2008
In a reserved parking lot outside the Erwin Center on Thursday night, a food trailer stands alert, its two walk-up windows open and buzzing with people placing orders and getting food. A trainer walks an elephant behind the truck. Pairs of miniature ponies and goats follow a little while later. A short, tall-haired guy with a big smile charges through. It’s Bello Nock, the “daredevil clown,” out of costume (except for the orange hair) and en route to prepare for the night’s show.
 
He pauses for a moment to talk about one of his favorite foods: shrimp. “I heard somewhere that flamingos are pink because they eat little shrimp all day,” Nock says. “I thought I could keep my color from going gray if I ate shrimp every day.”
 
The elephants? The elephants are one act that Ringling Bros. and Barnum & Bailey Circus food and beverage director Glenn Hunter doesn’t have to feed.
 
But feeding the 300-person circus staff is his job. He’s the man behind the so-called pie car — and the trailerlike pie car junior where everyone was congregating Thursday — on the circus’ mile-long train that travels nearly every day of the year to cities across the country.