My understanding is that a Deed of Grant is the document that provides you with the specific terms under which Rail Track have provided you with an easement over the piece of land in question. This is more than a normal private right of way easement - where one would be entitled to pass and repass over the land owned by Rail Track but not to park there. I imagine the Deed of Grant is the document that entitles you to do specifically this.
You will have to look to the Deed of Grant to determine what the specific terms are.
One would not normally expect Rail Track to be able to extinguish the Deed of Grant on a whim, and another document, a Deed of Variation is typically used to do this.
Were you properly advised about the terms of the original Deed when it was set up.
What exactly does it allow and for how long and what does it say about variation or extinguishment?