More accurately, trains and buses can be incredibly efficient per passenger mile.
Anecdotes are not data, but our local buses (city of ~50k) rarely have more than one rider, and a plurality of those I see have zero riders. In our case, these autonomous cars would be a much more efficient solution.
"More efficient than a bus system nobody uses" is a pretty low bar. In your city's case, shutting down the bus system and having everyone currently riding the bus take (potentially subsidized) taxis would be way more efficient, both environmentally and financially.
Anecdotes are not data, but our local buses (city of ~50k) rarely have more than one rider, and a plurality of those I see have zero riders. In our case, these autonomous cars would be a much more efficient solution.