" The reason for the 737's great success is its design flexibility. It lends itself well to modifications that fit the market needs of its customers, and currently, seven different variants are available.
The ability to order different versions of the same plane allows an airline to fit the airplane to a particular route and passenger load while maintaining a smaller inventory of support and service equipment for its fleet. And, like all of the planes in this family, the 737-400 has crew commonality with its siblings - a pilot qualified to fly one is qualified to fly all of them.
Though known as a classic, the -400 is no longer currently produced. It has been replaced on the production line by the -600, -700, -800, and -900, known as the Next-Generation Boeing 737s. "❞