Zorg2000: You could maybe try adding a solar powered spotlight nearby/separate from the camera, making the spotlight obvious and the camera more hidden.

A no-glow IR spotlight won't be obvious since it's not visible to the human eye, unlike a standard IR spotlight with its cylon glow telling you it's there.

Sorry to keep harping on about this one but I still think a trail camera would be the best solution in this case.  You don't have all the intelligence because they're running off the smell of an oily rag, but it makes up for it in capabilities (night vision, no-glow IR, runs for months unattended, camouflage, etc).  Also unless the gate sees more traffic than Britomart station I'm not sure if you'd need all the processing, there's either motion which is of interest or no motion.