Complex sentence: The HÉV is a hybrid between a train and a metro, since while most of its stations are overground (outdoor), some are underground (for example, the downtown terminus, Batthyány tér).
Split into simpler sentences: The HÉV is a hybrid between a train and a metro. Although most of its stations are above ground and outdoor, some are underground (for example the downtown terminus at Batthyány tér).