Complex sentence: Both Tilling-Stevens and Leyland produced single-cylinder prototype engines for the tank engine project and by 1959 the resulting complete engine design had become the Leyland 60, or L60, with the first engine running that same year.
Split into simpler sentences: Both Tilling-Stevens and Leyland produced single-cylinder prototype engines for the tank engine project. By 1959 the resulting design of complete engine had become the Leyland 60, or L60, with the first engine running that same year.