Railway-speak is a peculiar dialect, characterised by tortuous phrasing, a fondness for neologism, and a seeming reluctance to take responsibility for delays, unscheduled stops, failure to stop and indeed anything else that might go wrong. Announcers follow the principle that they should never use one word when two might do. Take, for example, the exhortation to “mind the gap between the train and the platform edge”. Why “platform edge”, and not simply “platform”? And, if you have to mention the edge, why do so in the odd phrase “platform edge” and not the far more natural-sounding “edge of the platform”?
The strangeness of the construction becomes clear when you apply it to other phrases. The Carpenters were standing “on the top of the world”, not “the world top”. And in the words of Kenny Price, everyone likes to be at “the front of the bus, the back of the church… [and] the middle of the road” – not the “bus front”, the “church back” and the “road middle”.
The rules governing the possessive and the use of noun combinations are incredibly complex, and there are exceptions – mountaintop, hilltop and table top sound natural, as do cliff edge and knife edge (although knife edge in particular seems more common as a metaphor). But, in general, possessives referring to a particular part or area of an object seem to use the “of” form (or, less frequently, “‘s”, as in “Land’s End”).
Here is an interesting discussion about the possessive and noun + noun combinations. It makes a distinction between objects belonging to living things (which typically take “’s”) and objects or parts of non-living things, where the possessive is formed through by using “of” or through a noun + noun combination.