Before I retired from working for BR/Railtrack/Network Rail the "Rules of the Route" document was only available to internal staff for engineering possession planning.
It contained a list of routes stating what dates and times a line or route may be blocked for an engineering works possessions. Such dates/times had been agreed by all concerned departments.
It would stress it is not a document showing when engineering possessions will take place only a document saying when they could be planned subject to conditions.
Thus it would show that route between A and C may be blocked any weekend between 1st October and 1st April. In practice the route would be blocked for say two Sundays plus one Sunday between A and B only.
Similiarly it could show route A and C could be blocked providing route D and E is open for diversions.
Other conditions that could be included are lines to be kept open for sporting events (major football matches, horse racing fixtures, The Boat Race in London, London Marathon,etc)
The document would also show the "white periods" which are times when there are no booked train services (either passenger or freight).
Finally it must be stressed that the document is only possessions are permissible (a Rule Book) as opposed to actually taking place.
