Complex sentence: Because the engine units are separated by the boiler unit, the weight of the locomotive is split over the two units, and they can run over bridges or line sections that might not be able to support conventional or Mallet locomotives of similar weight.
Split into simpler sentences: Because the engine units are separated by the boiler unit, the weight of the locomotive is split over the two units. Therefore they can run over bridges that might not be able to support conventional or Mallet locomotives of similar weight.