If you talk to a booster of government ("public") transit, you'll find that it's almost an article of faith that using it will cause less fuel to be burned, less pollution to be produced, and so on. I've got my doubts, and here's why.
A typical city bus that can carry 40 passengers gets about 3-4mpg, takes a somewhat circuitous route to get where you're going, and also requires its own garage, bus stops, and such. Overall, I'd estimate that the actual fuel efficiency of a bus comes out at 2mpg or less for miles actually traveled to one's destination.
In contrast, an ordinary car gets 24mpg on average. So unless the bus has an average (not just rush hour) of 12-15 passengers, the bus probably creates more pollution than driving to work. They also put hundreds of times more wear on the roads than passenger automobiles.
Now let's consider light rail. I've never seen fuel usage estimates, but one can guess from what we know about cars; 60% of the energy goes to fight wind drag, 20% to overcome rolling resistance, and about 20% for acceleration. The advantage of light rail is steel wheels; rolling resistance is greatly reduced. The disadvantage of light rail is that you need a lot more weight to keep those wheels on the track; typical carriage weights are around 50 tons.
So here's the estimate; about 5 times more wind drag, about the same rolling resistance (despite far heavier weight), but about 60x higher energy needed to accelerate the train than a car. This results in about 1.5mpg. After you account for energy used in transit stations and such, you're around 1mpg.
In other words, unless light rail is consistently over half full, there is no reduction in energy use whatsoever vs. that of a passenger automobile. Carbon emissions (given that you're burning coal to produce the electricity) are higher even if the trolley is completely full.
You could improve transit by going to hybrid vehicles, but then again, so can commuters. Overall, there does not appear to be a significant environmental benefit to using transit.
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