Yeah, the trains are totally fine as long as you don't mind no A/C (and avoid overnight trains). My dad was born in Gdansk and my mom's from Gdynia originally - Trojmiasto is really quite fun, and if you're there in July, you can go to the four-day outdoor music festival (Heineken Open'er). There's a beach that's fun to hang out at (being near the sea is AWESOME), and a lot of bars/clubs along the beach between the three cities (Gdansk, Gdynia, Sopot); there's also the Bar Przystan that you'll hae to check out - especially if it's a cooler summer day, get fish'n'chips there (fish is fresh) and ask for miod pitny (mead) or wino grzane (mulled wine). Super yum! If you move in the summer, tehre's a LOT of young people around, too.
Good luck!