Complex sentence: In fact, the Parnell Tunnel is composed of two tunnels, an older single-track tunnel and a newer double-track tunnel, though the older tunnel is closed off and has not been in use for most of the last century.
Split into simpler sentences: In fact, the Parnell Tunnel is composed of two tunnels, an older single-track tunnel and a newer double-track tunnel. However, the older tunnel is closed off and has not been in use for most of the last century.