I already practise this, I just thought I'd mention it since some inexperienced mappers put nodes literally everywhere without knowing how they really work (not even taking into account external addons)
Thanks you for taking your time and sharing all of this though

If you spend a little time to figure out how the nodegraph tool works it'd be really easy to remove the nodes you don't need yourself though, you'd have to extract the original map files and replace the gm_traintown.ain file with your node file also. It's really not very hard, just might take 30-60 minutes.

Ah, I see. I spend a lot of time on the rooftops myself though, and I have a mod that lets combine rappel from ledges so they need to be able to move around around up there to get to them. Plus, with that Better Half-Life 2 NPC Fights mod NPCs can jump around on certain areas up there, which they couldn't do without a navmesh/nodes.

Using those mods if you want a map to be swarmed with NPCs they tend to spawn not only on ground level, but isolated on all noded rooftops as well, just pacing back and forth. Take that overhang at the main entrance of the train station here for example... On small roofs (a few meters long) NPCs don't really need nodes to target you since you are so close and they'll just take a straight (literally) path to you. On the other hand, if you're on a huge roof that you can't normally access, well, you're not going to be there a lot, are you? But it takes up a lot of nodes and NPCs WILL spawn there
Anyway, for like literal sandbox when you spawn stuff around in the most unpredictable way possible these noded roofs might be of help, otherwise for gameplay they don't really bring a lot .-.

Oh really? What happens with those mods?

Unfortunately, those roof nodes are only a hindrance for mods like Zombie Invasion/Simple Horde Mode/RAIDS, and otherwise don't add much to NPCs' tactics