‘Model years’ were invented by car marketers, as a means of persuading people to change cars more often, not “for” either buses or trains.
Car makers naturally wanted to promote the idea that they used good engineering and their products were of high quality. However that implied that their cars would last for years, which was bad for repeat sales. They got round this by encouraging the idea of replacing cars because they were unfashionable rather than because they were worn out, and promoted that by making minor, often cosmetic, changes to their designs each year so that the car’s age was more evident. This had two main advantages for car manufacturers and dealers:
– more frequent sales of new cars without compromising their reputation for good engineering.
– a larger supply of second-hand cars, which led to more people being able to afford motoring and so to more political support for it. In the early days of the car, they were seen as a self-indulgence by the wealthy and so there was opposition to spending public money on highways, etc, for them, which in turn limited their usefulness. The spread of cars into lower income groups reduced that.