Complex sentence: However, the first diesel locomotive, the "Dongfeng", was introduced in 1959 and during the 1980s and 90s, diesel and electric locomotives gradually replaced the steam engines on main lines.
Split into simpler sentences: However, the first diesel locomotive, the "Dongfeng", was introduced in 1959. During the 1980s and 90s, diesel and electric locomotives gradually replaced the steam engines on main lines.