Complex sentence: It is not necessary to match the power available to the size and weight of the train--each unit is capable of moving itself, so as units are added, the power available to move the train increases by the necessary amount.
Split into simpler sentences: It is not necessary to match the power available to the size and weight of the train, as each unit is capable of moving itself. Asunits are added, the power available to move the train increases by the necessary amount.