Complex sentence: The X4500 class are one of four classes of similar design (X4300, X4500, X4630, X4750) known as "Caravelles" as when built the engine sound reminded railwaymen of the contemporary French SE210'Caravelle' jet airliner.
Split into simpler sentences: The X4500 class are one of four classes of similar design (X4300, X4500, X4630, X4750) known as "Caravelles". This name comes from when built the engine sound reminded railwaymen of the contemporary French SE210'Caravelle' jet airliner.