Board Thread:General Discussion/@comment-26873653-20160723035910/@comment-679780-20160827202851

We know Classic Sonic and Classic Tails know each other but there is little evident he knows his Modern friends. Such as when Knuckles calls him fat, to him was some dude he rescued judging his gut.

Since there was no Classic Knuckles, we can pressume the Classic characters come from somewhen between the Classic eras. Sonic CD takes place after Sonic 2 but since Sonic 2 and 3 transit into one another we can pressume Sonic CD takes place somewhen else, the manuals state Sonic decided for Tails to stay behind but Knuckles was not mentioned.

Plus after Sonic Generations, Sega re-released Sonic CD to have Tails playable.