Complex sentence: During the mass withdrawal of the W classes in 1994--96, the majority of this class was retired in preference to the higher class-Ws, this was due to the discovery of asbestos in the controllers.
Split into simpler sentences: During the mass withdrawal of the W-classes in 1994--96, the majority of this class was retired in preference to the higher W-classes. This was due to the discovery of asbestos in the controllers.