Complex sentence: This system was commenced by the MDR for its stock, and when London Underground was formed, the ex-Metropolitan Railway units were fitted into this series (types MV, MW, T, V, VT and W).
Split into simpler sentences: This system was commenced by the MDR for its stock. When the London Underground was formed the ex-Metropolitan Railway units were fitted into this series (types MV, MW, T, V, VT and W).