Complex sentence: In 1955, Buick and Oldsmobile introduced the first four-door hardtop sedans, and in 1956 the first four-door hardtop station wagon was introduced by Rambler.
Split into simpler sentences: In 1955, Buick and Oldsmobile introduced the first four-door hardtop sedans. In 1956, the first four-door hardtop station wagon was introduced by Rambler.