Complex sentence: The 1972 Munich Olympic Games presaged a major expansion of public transport, with the creation of an S-Bahn network and the extension of the U-Bahn, both carried in new tunnels under the city centre.
Split into simpler sentences: The 1972 Munich Olympic Games presaged a major expansion of public transport in the city. In 1972 a new S-Bahn network opened that, like the U-Bahn, was carried in new tunnels under the city centre.