Interesting that we worry about empty stock and contra peak trains, but no one does the same for cars. For example I know someone who drives her kids for 45 mins to a private school, despite good local school within walking distance, then does an "ECS" journey home and then reverse for the evening collection. Journeys like this a driving the need to widen the A414! How many cars run most journeys only 25% occupied? (Yet ride a tandem bike to pick up a child and the same drivers ask you if someone fell off!)
Indeed, is also point out that few people driving their small cars would be able to achieve 150g/km on a lot of journeys, especially those where there's congestion.
There's also a load of fairly unproductive journeys in cars, such as getting fuel, getting the car serviced, driving around to find a parking space, going and looking at new cars etc. Although there will be those for whom such trips are fairly small (such as only getting fuel whilst at a supermarket, live/work within a short distance of a car maintenance garage, have their own driveway, etc.) these all add up and aren't included in the general milage figures.
As such there's always going to be examples of where the CO2 emissions are going to be worse than the quoted figures.
If also point out that if I go by train on a service which is already running the amount of extra CO2 produced will be very small, whilst if I drive it's all extra. There's also scope for some of the journey to be undertaken by other modes, such as walking or cycling, which wouldn't be as easy when driving (to the extent that few do so).
Of course all this discussion so far had been looking at the use of the vehicles and not the construction in the first place. Although rail had a lot of CO2 required to build the tracks, typical payback periods are 10-30 years. Likewise with most trains only being scraped after 30 years, whilst most cars have a much shorter lifespan.
As an example a typical European car can have something like 250g/km when including lifetime emissions, whilst fairly efficient cars can be around the 170g/km. However even that doesn't account for road maintenance emissions, which as road surfacing and pairing white lines involve a lot of heat so are hardly very green.