Add emission factors for rail modes

[rafapereirabr]

There is some info on rail emission factors in this report by the UK government.

• Defra (Department for Environment Food and Rural Affairs) and DECC (2013) 2013 Government GHG Conversion Factors for Company Reporting: Methodology Paper for Emission Factors. Defra and DECC, London, UK

They also have a more recent version of the report for 2021.