train from lisbon to porto

Nicholas, The Golden rule is go to the website of the company running the trains, only use another site if you have problems (refuses your card).

Rule 2 - look it up on the "Man in Seat 61" website. This is his entry on Lisbon to Porto: https://www.seat61.com/international-trains/trains-from-Lisbon.htm#Lisbon_to_Porto

The Portuguese Railways website https://www.cp.pt/passageiros/en sells tickets online for Portuguese long-distance trains including Lisbon-Porto . . . .

Definitely use the official website - https://www.cp.pt/passageiros/en/buy-tickets

FYI, while you will save some money on the fast AP or IC trains if you buy in advance, you won't save nearly as much vs. last minute compared to countries like France and Italy when the fast trains are very expensive at the last minute. I didn't book any trains in advance before my trip to Portugal in 2018. I think it might have cost about $35 USD for a one-way ticket instead of about $15 if booked in advance. Given that my itinerary was not fixed ahead of time, it didn't make sense for me to be limited with an advanced booking.

By the way, when taking the train from Lisbon to Porto, you must change at least once. All trains to Porto include the short train connection from Porto Campanha (where the fast trains stop) to the smaller Sao Bento station at the center of town. In Lisbon, you'll need to get from the center to one of the stations the fast trains use like Oriente or Santa Apolonia station. It may make more sense to take a metro or bus (or Uber) from your lodgings in Lisbon to one of those two stations (I took the metro to Santa Apolonia). Trains in Lisbon are a bit odd - the main stration stations are not well connected to each other. Rossio station is very convenient for a direct train to Sintra but not to the other main train stations.