Complex sentence: The T.G.R. was the only system to use this class, the idea being that this class would replace steam locomotives on the T.G.R., but this did not eventuate for another 25 years.
Split into simpler sentences: The T.G.R. was the only system to use this class. The idea was that this class would replace steam locomotives on the T.G.R., but this did not eventuate for another 25 years.