Complex sentence: The Central Subway extension is seen as a precondition for future light rail transit along the heavily used Geary corridor, because the Central Subway will provide much of the downtown, subterranean infrastructure that a light rail system along Geary would require.
Split into simpler sentences: The Central Subway is seen as a precondition for light rail transit along the heavily used Geary corridor. This is because the Central Subway will provide much of the downtown, subterranean infrastructure that a light rail system along Geary would require.