Whilst we should be doing more electrification as a network the per passenger km rate is 31g of CO2, because a lot of passengers are using electric trains (chiefly DMU's are rarely over 4 coaches long when in service, when that's almost the minimum for EMU's).
A car with emissions of 100g/km would need 4 people to match that, however cars average 1.5 (although the average is 167g/km)
An EV has emissions of 46g per km (2025), so only just edge rail if they maintain an average of 1.5 people per km traveled.
Emissions per km for cars from here:
The other factor is, whilst people do use diesel only trains is also not uncommon for rail passengers to also use electric trains (as well as walk/cycle) so as an individual rail users are more likely to have a lower carbon footprint than those who own a car.
Especially given it's not uncommon to see people driving distances where the time saving is very limited (if not actually slower on a door to door basis, rather than engine on to engine off basis).
There are still some fairly easy wins for rail to deliver. For example XC could stay the transition to a bimodal fleet (I've previously suggested 12 nine coach 80x's whilst converting their current 22x fleet to being 43 six coaches, as that would get a big uplift in capacity without scraping too many 22x coaches or increasing the number of units on lease by very much), even if the 22x's are used as longer trains the emissions per passenger (assuming passenger growth, which was 15% last year and 18% the year before) is likely to reduce, as the emissions to run a service as 4+4 is less than two separate 4 coach units.
It's also worth noting that a 6 coach 22x units would have something like 332 seats vs 400 seats for a 4+4 train, even though there's 2 coaches fewer.
By having 6 coaches you could even reduce the fuel burn by (if suitable) removing the engines from one of the 6 coaches (a more expensive option would be to replace with batteries, however that would reduce fuel burn further), although at the very least it would allow engines to run a bit less hard.
However, that gives XC their first bimodal units (even if it's a fairly small percentage of their fleet), which could then allow then to be scaled up as time moves forwards. The cost of reforming the 22x's would be fairly limited and so wouldn't be too wasteful if they were then started to be withdrawn from service in (say) 8 years time to allow more bimodal trains to enter service.
8 years would be 2033 and so people can still buy plug in hybrids but not pure petrol/diesel cars and whilst new cars maybe mostly electric, there's still going to be a lot of petrol/diesel burn as part of the car fleet.
Whilst XC is only a small amount of the DMU fleet, as they are high speed and often with low numbers of seats per train (and certainly a low number of seats per coach) any reduction from them maybe enough to have an impact on the per passenger emissions at a national level.
