I would guess that it's to do with the Pullman company.

Basically, back in the 19th century, long distance rail travel was uncomfortable due to poor, if any, sleeping accomodations. Pullman developed a business with very successful sleeping cars. Anyway, to further improve passenger rail, dining cars were developed so that the train didn't have to stop periodically so everyone could get out and eat. Because trains are fairly narrow, and you need enough room for people to pass through cars so they can walk the length of the train, this necessitated putting the kitchen appliances, cabinets, etc. on one wall of the car, in the kitchen area.

Another interesting note: the reason that classic diners often look like trains, was because there was an attempt to capitalize on the popularity of railroad food.