Train from Porto to Lisbon.

It's pretty easy but a little more hassle than in other countries, because you have to change trains at least once maybe twice to get from city center to city center. You'll probably be near the Porto Sao Bento station, and the fast trains don't depart from there; all train tickets to Lisbon would include the short train ride from Sao Bento to Campanha station. From there you'll take a fast train (AP is fastest, IC a little slower but a little cheaper). You'l like get off that train at Lisbon's Oriente or Santa Apolonia station and from there get to your destination in Lisbon. Lisbon' has numerous train stations that are not well connected together - getting from Oriente or Santa Apolonia to your destination by metro train (or Uber or something) is probably easier than taking a connecting train - I used metro. E.g. Rossio train station in the center of Lisbon is really convenient only for making a day trip to Sintra - it's generally easier to use the metro train to get to the other train stations.

I didn't buy my train tickets in advance. Yes, you can save a little money by buying in advance but you won't save as much as you would in other countries like Italy or France. If you can fix a time / date in advance, then maybe you can buy your ticket ahead of time. I did a lot of train travel not a direct train between Lisbon and Porto, and I didn't have a fixed itinerary ahead of time.

More info here on the official website:

https://www.cp.pt/passageiros/en/buy-tickets

We did this run in 2014, and it was about as easy as you can imagine. Buying the ticket at the Sao Bento station was simple. There was no line at the window, and the whole transaction was conducted in English. The best option available on our day of travel was a Regional/Intercity train, so there were a few stops and no high speed option. First Class either wasn’t available or didn’t seem worth it at the time. Can’t remember if we had to transfer at Campanha Station. We met some interesting local folks, and overall it was an enjoyable ride. Make sure you take time to enjoy the tiled interior at the Sao Bento Station.

The train was a bit worn, but clean and efficient. It ran at a rather high speed, something like 140 KPH.

The train is inexpensive so we did first class, it was comfortable.

The AP (fast) train leaves from Porto-Campanha. It is much easier to take an inexpensive taxi or an even less expensive Uber from the city center. It's not worth the hassle of getting to the city train station and taking a local train to Campanha. All the trains stop at Oriente station in Lisbon, some continue to Santa Apolonia. I chose to continue to Santa Apolonia and waited a long time for a taxi. Where you get off the train and how you get to your hotel depends on where your hotel is.

The AP trains are a little faster than the IC trains, and they are newer so the ride is more comfortable. Second class is fine, 1st is nicer. It's less than 3 hours on the train.