Nowadays if you are an airline operating within the US domestic market and you wish to serve a route pair you simply do just that, after relatively little fuss and bother, but prior to 1978, when the US market was regulated by the Civil Aeronautics Bureau (CAB), starting new routes was a marathon process. Indeed, most of the time getting permission to start a new route, especially if there was any competition involved, was a non-starter. Whence the regulated period threw up a selection of idiosyncratic practices that made sense then but look a bit weird today. One of these was the concept of interchange services.
The original 737-100 and 200 were not big sellers for Boeing in the 1960s even in the USA. Western was renowned for going against the norm having not operated DC-3s or 727-100s so the 737 was in a way a natural fit and the airline's choices were limited after it was forced to cancel its ten orders for the One-eleven. As with the 720 the 737-200 would go on to be a major component of the fleet and unlike the former it was never replaced, except by more 737-200s (the Advanced model).
I'm Richard Stretton: a fan of classic airliners and airlines who enjoys exploring their history through my collection of die-cast airliners. If you enjoy the site please donate whatever you can to help keep it running: