2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 1, Chapter 8.5, https://www.ipcc-nggip.iges.or.jp/public/2006gl/
Definition:
The CRF sector (IPCC 2006) 'railways' is a transport sector defined by the 2006 IPCC Guidelines for National Greenhouse Gas Inventories:
Emissions from railway transport for both freight and passenger traffic routes.