Complex sentence: The first ETD use is attributed to Florida East Coast Railway in 1969, soon other Class I railroads began using ETD's as well, until the mid- 1980s when they were common equipment.
Split into simpler sentences: The first ETD use is attributed to Florida East Coast Railway in 1969, soon after which other Class I railroads began using ETD's as well. By the mid- 1980s when they were common equipment.