Ex-Ford engineer Richard Parry-Jones is now chairman-designate of the UK's Network Rail. Having looked into the futures of both car and train development, he believes that by the end of this decade the most Earth-friendly internal-combustion engine cars will be about as polluting, on a per-passenger basis, as high-speed electric trains.
Parry-Jones says carmakers are targeting 40g/km of CO2 tailpipe emissions by 2020. If the "average" occupancy of 1.6 people-per-vehicle stays the same in eight years, that would equal 25 gm per person per kilometer and put such cars in the same environmental category as the cleanest mass transit.
Since the emissions of electric cars and trains is usually given as zero, we're assuming he means diesel-electric high-speed trains; life-cycle emissions wouldn't be a valid comparison since those aren't factored into a car's tailpipe number. According to The Guardian's table of emission numbers by transport, right now you'd have to load four folks in a small diesel topped up with ultra-low sulphur fuel to get down to 42 grams per passenger per kilometer (gpkm), which would put you right with one of Virgin's Voyager class diesel-electric trains at 75-percent capacity. If you want to go all the way low, though, you'll need a 50cc two-stroke and a passenger: at that point you're rocking just 19.5 gpkm. Slowly.