Complex sentence: A proof-of-payment fare system is in force on all S-Bahn trains; there are no fare gates, but those caught without a valid ticket during a random ticket inspection face a minimum fine of CHF 100.
Split into simpler sentences: A proof-of-payment fare system is in force on all S-Bahn trains. Fare gates are not used, but those caught without a valid ticket during a random inspection face a minimum fine of CHF 100.