Help: Looking multiplayer map made at least 19 years ago :O

Me and a group of friends used to LAN Half-Life 1 deathmatch and Sven Coop on this one map a lot waaay back in the "good old days". At least 19 years ago at this point.

We were thinking of trying to get together and crank out some LAN action again soon, and we thought we might plumb the local experts for this map we've been hunting for. We don't remember the name of it and all we have is a description:

The map was massive and entirely in-doors - Probably one of the biggest multiplayer maps we'd played. It had like one side with a massive drop to a black abyss and a lot of hand-railed walk ways hanging from the roof, then an office area with lots of vents and stuff to crawl through, and then another side with a concrete floor and working mono-rails and some sort of heavy machinery, and very distinct lighting for each area, the industrial machine area with the concrete and monorails was lit red, the office was bright normal lighting and then the area with all the walk was was blue-ish.

I'm deeply hoping someone here gets a fond memory and can help out. Also, what's the best place to find these old maps? Moddb doesn't seem to have that many in total.

Is the map you're looking for official or unofficial ones (custom map) ?

Because the official Half Life deathmatch map with mono-rails that I've known was called "Subtransit" or "op4_park" if you're playing Half Life Opposing Force