Hello Floriano & welcome to the forum
Rail Accident Investigation in the UK is carried out by the Rail Accident Investigation Branch, which is part of the Department for Transport, which itself is a Government Department. Website here at ........
http://www.raib.gov.uk/publications/index.cfm
Their website includes an insight into recent accidents/incidents.
However I don't know whether you'll find any manuals of
HOW to investigate. Accident Investigators are usually recruited from people with relevant technical or investigatory experience, or from within the Rail Industry according to their experience, eg Operational managers who have been promoted from the train driving grade, or engineers, etc.
RAIB investigators are usually qualified to degree or chartered engineer level (YES - there are train drivers who have degrees !).
HTH, Flying Pig