Lisbon-Porto train

1. I'm about to buy Lisbon-Porto ticket, but cp.pt forces to enter return date.

When I recently bought my ticket on cp.pt, I simply left the "return date" field blank, then hit submit and it gave me one-way train ticket options and prices.

2. I'm in Rossio area, so I select Lisboa-Rossio.

This was confusing to me as well because, if you are buying a ticket for an AP (Alfa Pendular) train, eventually your ticketing option may say show "TC" (Transporte Complementar) for the leg from Rossio to Lisbon Santa Apolonia. That sounds very much like it's "complimentary" - aka free.

I stopped in at the train station in Lisbon to ask what the "TC" refers to. Basically, it means you're on your own to find service to complete the leg of your trip from Rossio to Lisbon Santa Apolonia. And, as helpful forum posters informed me in this post, there is no train that runs from Rossio to Lisbon Santa Apolonia.

I took the metro from Restauradores to Santa Apolonia which was fast and easy, though I did have to navigate some stairs with luggage. The metro brings you up right at the Lisbon Santa Apolonia train station. You can also take a bus or Uber.

Even if Porto Campanha station seems easier or quicker to get to your hotel, please take Gail's advice and transfer to a regional train for that last leg from Porto-Campanha to Porto-Sao Bento. It is a stunning view! (I sat on the left side of the train to see the city center area as the train approached.)

You might also like to take time to view the Sao Bento station while you're there, which is beautifully tiled and a sight in its own right. But that may be easier to walk back another time after you've dropped off luggage.

3. Upon boarding the train, will train go direct to Porto, or will I have to transfer to another along the way?

This will depend on which ticket/train/routing you buy. Some tickets may begin at Lisbon Santa Apolonia and require you to change at Lisbon Oriente. Regional trains may include transfers at other stations. You can view the details in the "Train Times" section prior to choosing which ticket to buy.

I agree, Gail, the stunning view is worth the wait for the regional train!

P, I see in your profile that you plan to travel in the spring or fall of 2023. Tickets are only available 2 months in advance, though I applaud your advance planning.

In buying my tickets for my just-ended trip to Portugal, I learned that the "Promo Ticket" discount fares are available until 10 days in advance. It's not like airline tickets where the price may continue to increase as the date approaches. If you are buying a ticket with a reserved seat, you may wish to check availability of your preferred seating type to decide when to buy.

I appreciate your responses.  I'm on only my third day in Lisbon and have covered a lot of ground. By making mistakes, such as getting lost, missing my metro/bus stops, or misunderstanding directions, I've discovered areas that I hadn't meant to visit but found as charming as previous ones. I have no set agenda and just winging it.  I find not many Portuguese (outside hotels and restaurants) understand English and my phonetically pronounced Portuguese is nowhere close to how locals say things; it's been a bit challenging. My elementary Spanish and Latin studies have helped me somewhat understand directions and signs.  I sense the Portuguese are a little offended when I try to communicate in Spanish, though.

Thank you for your advice on trains to Porto.  I will take metro (linha azul) from Restauradores to end-of- line at Santa Apolinia, then take AP (express) train to Porto Campanha, then hop on to local train to Porto Sao Bento, sitting on scenic (left) side for views.
It was explained to me too that AP is the express line and gets to Porto in 2hrs 46 mins or less, while non-AP schedules get there in 3hrs 15 mins, stopping several times and may require changing lines.  
Only 4 more days in Lisbon to get in two day-trips: Sintra and Cascais. Then off to Porto! I'm getting fairly proficient with old-town Lisbon and feel almost like a local.
I appreciate all your advice. Thank you!
p.s. CWsocial - I'm actually travelling now - had to book quick. Use or lose vacation before year-end. Thought it'd be rainy but it's only rained overnight - haven't seen a drop during the day. It's been wonderful but rather warm at 68-70.

If you have time and enjoy seafood, I suggest a side trip to Matosinhos. It's totally off the tourist track, but oh so worth it. Restaurants set up grills along R. Heróis de França in the evening. The seafood is fresh, and inexpensive. If memory serves right, I paid €9 for a main meal. It included the seafood option I selected, plus sides. Add a glass of wine and it's still less than €15. Most restaurants offered covered seating, which makes this work even in the off season.

I took a local bus from Porto and back, fun to watch the locals and the scenery.

Yeah, I think you’re better off with English and google translate in portugal rather than speaking Spanish! I found it really easy to get along there and I thought a lot of people spoke English but ymmv 🤷‍♀️