Earlier quoted context omitted.
>"Diesel buses are very energy efficient. Electrified rail is surprisingly inefficient." I would really like to see a citation, because I find that extremely hard to believe.
Really depends on ridership. There's also pollution other than just energy inefficiency; old diesel buses with low ridership put a lot of particulates and other pollution into the air, compared to the same number of passengers in the newest petrol (or cleaner diesel, or obviously electric) cars.
But you also have to factor in that most buses run for a lot longer than the average car, sometimes decades and millions of kilometers. Most of the time they're not even "put to pasture" because they're worn out, but because newer buses are more efficient and have more creature comforts. So the older buses are brought in when for instance there is rail maintenance and you have to replace a train line with buses. In comes the old stock, still pulling its weight.
This also ties in to the environmental load of producing new cars/buses, you need a lot less raw material to make a bus that moves 60 people, compared to the cars needed to move 60 people.
That's why old stock is cycled out regularly, to improve efficiency and reduce pollution on the busiest lines. The the older buses get moved to more sparse routes, with less busy schedules and less stop-and-go.
Where I live, a surprisingly big problem is actually that too many people ride the buses during rush hour. Sometimes you just have to wait for the next bus to come and hopefully have some space for you. I'll admit that's really a luxury problem to have.
Public transport is vastly better for the environment and for congestion, unless nobody uses it. That takes investment, but politicians are way too quick to say "well no one's using it now, why should we invest?".