you may find the following govt report on the carbon footprint of UK rail has some relevant numbers. 

Headline figures.. passanger trains dominate the ‘leccy bill’ at  just under 4000GW hr per year.

Freight is a lot less electric.

In 2018-19 passenger services consumed:3,976 million kWh of electricity Increase of 9.1% compared to 2017-18  469 million litres of diesel Decrease of 5.3% compared to 2017-1

in 2018-19 freight services consumed:75 million kWh of electricity Increase of 12.7% compared to 2017-18  153 million litres of diesel Decrease of 6.7% compared to 2017-18

Mike 

PS edit.

Another source of useful figures is the 2012 Ricardo report into the efficiency of diesel trains.in 2012 one of their recommendations was very similar to the sort of thing in a modern car.

Technology Package #6 (idle stop/start, gas exchange system upgrade and small genset for freight locomotives) –Overall this is the most attractive retrofitting solution of those considered for improving the efficiency of the GB rail fleet by applying it to all 450 Class 66 freight locomotives–TP #6 consists of an idle stop/start system to improve idle fuel consumption, a small diesel genset to provide cabin comfort when the traction engine is off and a gas exchange system upgrade to improve higher load fuel efficiency–The payback period is under 1 year and it could save 89 million litres of diesel per year which equates to savings of £53 million per year at £0.60/litre of diesel.

As far as I know, this has not been adopted

you may find the following govt report on the carbon footprint of UK rail has some relevant numbers. 

Headline figures.. passanger trains dominate the ‘leccy bill’ at  just under 4000GW hr per year.

Freight is a lot less electric.

In 2018-19 passenger services consumed:3,976 million kWh of electricity Increase of 9.1% compared to 2017-18  469 million litres of diesel Decrease of 5.3% compared to 2017-1

in 2018-19 freight services consumed:75 million kWh of electricity Increase of 12.7% compared to 2017-18  153 million litres of diesel Decrease of 6.7% compared to 2017-18

Mike 

PS edit.

Another source of useful figures is the 2012 Ricardo report into the efficiency of diesel trains.in 2012 one of their recommendations was very similar to the sort of thing in a modern car.

Technology Package #6 (idle stop/start, gas exchange system upgrade and small genset for freight locomotives) –Overall this is the most attractive retrofitting solution of those considered for improving the efficiency of the GB rail fleet by applying it to all 450 Class 66 freight locomotives–TP #6 consists of an idle stop/start system to improve idle fuel consumption, a small diesel genset to provide cabin comfort when the traction engine is off and a gas exchange system upgrade to improve higher load fuel efficiency–The payback period is under 1 year and it could save 89 million litres of diesel per year which equates to savings of £53 million per year at £0.60/litre of diesel.

As far as I know, this has not been adopted