Crafty members of the community have been digging through the game's files and found all those discarded classics. What's more, GT3 tracks are in there too, and even the PS1-era Pikes Peak track!
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The method to experience these tracks is what we'll call… "unofficial". To boil it down, it involves tricking the game into thinking it's loading one track, when it's actually loading one of the hidden circuits. With the GT5 online servers switching off years ago, this doesn't hurt any players, but does allow for fans to experience old tracks with cars they've never been able to before.
also, bridal cars :