User blog comment:WildBrick142/Old Subway Tunnel/@comment-5022113-20120422060856

Go to Youtube website and search "gta lcs secret tunnel". Then look at the first video, it shows you how to get in the "secret" tunnel (The unfinished Porter Tunnel). If you haven`t done "Bringing the House Down", you can use the glitch but you can`t go to the abandoned subway tunnel. If you`re doing "Bringing the House Down" now, you can explore the abandoned subway if you not have planted the first bomb in the mission. If you have done the mission, you can`t go to the abandoned subway anymore.

Good Luck!