u/Appropriate-Dish-466

Visiting Gdansk with my 4-year-old soon and I'm finding conflicting information whether or not I need to buy him a public transport ticket. Is it free for under 4 year olds or 4 included? I will be using Jakdojade app and I would just need to buy him the reduced fare ticket right? When taking the train to Sopot, in that case he would also need a ticket right?

Thank you!