Complex sentence: The vast majority of BREL's output was for British Rail, including Mark 1, Mark 2 and Mark 3 carriages, the latter for locomotive haulage and InterCity 125 diesel High Speed Trains.
Split into simpler sentences: The vast majority of BREL's output was for British Rail. These included British Rail Mark 2 and British Rail Mark 3 carriages, the latter for locomotive haulage or InterCity 125 diesel High Speed Trains.