Barney and Tracy's first meeting is way before they actually become friends. In Platonish, when Lily and Robin challenges Barney to pick up a woman while buying diapers and Samosas, he tries hitting on Tracy. They talk on a bench for a bit, and Tracy convinces Barney to finally go and get Robin. It's their discussoin that prompts Barney to write "The Robin". They do not speak to or see one another again until Barney and Robin's wedding reception, where Tracy is the bass player in the wedding band. Barney tries introducing Ted to Tracy, but they meet later at the train station.