Complex sentence: The "Hokutosei" overnight train service began after the completion of the Seikan Tunnel, and a later and more luxurious "Cassiopeia" overnight train service was often fully booked.
Split into simpler sentences: The "Hokutosei" overnight train service began after the completion of the Seikan Tunnel. The newer and more luxurious "Cassiopeia" overnight train service is often fully booked.