On 27 June, my family will travel from Varenna-Esino to Chiasso on R2817 and RE80. Then, we will board EC178 and PE2928 to get to Interlaken Ost (connection in Luzern). We have purchased Swiss Travel Passes. Do we need to make seat reservations or do we simply board the train and assume we can find 4 seats together? Thanks in advance for your help.