Not to be confused with Drainage12, this is a 4 map campaign.
The author made an update to the maps for the improved engine that cause some of them to not work properly on the older. Luckily, I had saved the old version.
These maps have no issues though the combine soldier that you have to sneak past may be alerted if he is spawned with a shield or visor. Shut them off on the first map if you are using them or you may have an impossible challenge ahead of you.
All credits go to Remonttimies.
http://www.mediafire.com/download/3m7xu21xukj6iad/drainage2k6.zip