Travelling between the three cities of Gdańsk, Gdynia and Sopot is a cinch, and best done using the SKM kolejka (commuter train). Trains run more or less every 10 minutes between 05:00 - 19:00 through the Tri-city and less frequently outside of these hours with trains also running occasionally between 00:00 and 04:00.
Tickets can be bought from modern ticket machines which can be found on most platforms and have instructions in English and a couple of other languages. Alternatively you can buy tickets in main stations where you see the sign for Kasa Biletowa or as a last resort from the conductor at the front of the train. If you buy tickets from the Kasa Biletowa make sure that you stamp them with the time in the bright yellow boxes on or close to the platform to validate them.
Ticket prices are dependent on the number of stops you are travelling but a journey from Gdynia to Gdańsk will cost 5.40zł and take about 35 minutes (Sopot will cost 3.60zł and take about 15 minutes from Gdynia). Bikes travel for free and should be carried in a specially marked carriage usually at one end of the train. Children under 4 travel for free although you also have to buy a 0zł ticket (we kid you not) and you are likely to be requested to provide proof of the child‘s age so basically keep passports on you at all times. ISIC cards will get you a student discount but only if you're a Polish student, and Euro 26 cards are not valid and you will have to pay a fine if caught travelling without a validated ticket.
Trolleybuses, trams and ordinary buses (ZKM/ZTM) can also zip you around the relevant cities. Tickets are issued by Gdańsk and Gdynia. Gdansk tickets allow you to travel in Gdansk (and Sopot) and Gdynia tickets allow you to travel in Gdynia (and Sopot). If buying a ticket in Sopot you will need to buy a ticket for the direction you are heading i.e. Gdańsk or Gdynia.
Single - 3.00zł
60 minute ticket - 3.60zł
Single ticket for use on a ‘fast’ line - 4.00zł
60 minute ticket for ‘fast’ lines - 4.60zł
24-hour ticket - 12zł
If you‘re planning on staying longer then there is the Metropolitan ticket and covers all 3 cities for 72 hours - the first type covers buses, trolleybuses and trams and costs 28zł (14zł reduced) or a second version which also includes SKM trains with this one priced at 40/20zł. Tickets can be bought from kiosks or an increasing number of ticket machines which have English and German language options.
A recommended way to avoid having to decide on which ticket and then having to negotiate how to get it with the woman in the kiosk is to pick up a Gdansk – Sopot – Gdynia Plus tourist card. As well as the discounts and free entry to many of the sights that the card brings, you will also be entitled to free travel on the whole Tri-city transport network. Pick it up at the tourist information points around the city.