Complex sentence: The mini-buses were started in the late seventies as a relief for the office commuters from the overloaded buses by being a sitting-only service, but this has given way to severely overloaded and crowded mini buses.
Split into simpler sentences: The mini-buses were started in the late seventies as a relief for the office commuters from the overloaded buses by being a sitting-only service. But, this has given way to severely overloaded and crowded mini buses.