Network Rail and the maintenance company Jarvis Rail are to be prosecuted over the 2002 Potters Bar rail crash in which seven people died.
The Office of Rail Regulation said it had started criminal proceedings against the companies "for breaches of health and safety law which caused the Potters Bar derailment".
If found guilty both companies could face unlimited fines. The prosecution follows the end of the inquest into the deaths. The jury decided poor maintenance was the root cause.