Reduced carbon emmissions at source.
You don't need the "at source" bit, maybe 5 or now years ago, however there's been some significant changes to the way our electricity is generated so that it is significantly less reliant on fossil fuels.
First up we now use ~15% less electricity than in 2010, with it being the lowest since 1994 and the lowest per person since 1984.
Next the use of coal has fallen from 30% in 2014 to 5% in 2018 (which is actually a bigger fall than those percentages indicate as the amount of electricity being used has fallen). With gas, although it has increased a little since 2014 it is still a lot lower than in 2010.
Renewables have already exceeded the predictions made in 2010 for 2020, with more due to come online in the next few years.
Of course we are still a long way short of where we could be, however there's still a good environmental case for the electrification of the rail network.
The other thing to bear in mind is that there can be advantages to the traveling public from electric traction over diesel trains. Although there's some doubt over the actual impact from the "sparks effect" due to it also often resulting in increased in frequencies which increase rail use anyway regardless of what type of power is used. The advantages tend to be:
- there can often be an increase in capacity due to the length of electric trains (typically 4 coaches) trending to be longer than diesel trains (typically 2 or 3 coaches)
- the trains are quieter
- the trains are more pleasant to be near (no diesel fumes)
- the trains are cheaper to lease, fuel and maintain, as well as there being a much larger overall fleet size, which makes it cheaper and easier to lengthen trains when they get busy (up to the maximum length which the routes can accommodate)
- is easier for people to see that they are doing the right thing environmental
Although these may not increase passenger numbers as much as increasing frequencies they are all going to have a small impact.
If they create enough of an impact then it could then lead to an increase in frequency.