I could write screeds on this but, to keep it brief, when the EU and everybody else decided to take railways out of the public sector and bring competition to rail, the first step was to split the former nationalised companies into EF entreprises ferroviaires and GI gestionnaires d'infrastructure (railway undertakings/infrastructure managers). In fact the idea originally stemmed from the railways themselves wanting séparation comptable (accounting separation) so that people could see that their business side (passengers/freight) was a going concern but that it was the cost of infrastructure that was dragging them down (unlike the roads that were and still are state-subsidised).
This latter idea can be traced back to a thing called the " Lima Declaration", for anyone really interested.

In fact, it all backfired rather and the upshot was privatisation. And with privatisation came the need to manage the presence of more than just the historic operator on the national rail networks. The GI/IM (RFF in France, for example) has to allocate available space among bidders in the particular country and this available space is called "train paths"


"Qu’est-ce qu’un sillon ?
C’est la capacité d’infrastructure requise pour faire circuler un train donné d’un point à un autre à un moment donné (équivalent au créneau dans le transport aérien)."
"What is a train path?
A train path is the infrastructure capacity required to work a given train from one place to another at a given moment in time (akin to a "slot" in the air transport sector)."
(taken from RFF literature)

--------------------------------------------------
Note added at 20 hrs (2007-06-17 08:01:33 GMT)
--------------------------------------------------

FYI - to my great shame, my "glossary" is pages and pages of handwritten stuff, compiled over the years. I always said I would put it all on computer but it is SUCH a boring task.....