Train Travel

Tickets for the Sintra to Lisbon Urban trains (commuter train) aren't available online.
You'll need to purchase the Navegante Ocasional card and load it with either money or tickets.
You can purchase the card in person at the train station either at the maned ticket booth or the ticket machines.
The Navegante Ocasional card is also valid at the Lisbon to Cascais train , Lisbon Metro, Lisbon buses ( Carris), Lisbon ferry, depending if you to choose to load money or tickets into the card.

Tickets for the AP train are available online a maximum of 60 days before departure, usually the earlier the bigger the discount.
Senior discount isn't available on discounted tickets. Sometimes the senior discount is higher than the early purchase discount.
Discounts usually can't be combined ( local residents have ticket options not available to tourists , in case you see it online).
The discount is available in both first and second class in the AP train.

I've had issues with accessing the online tickets so I always carry a printout.

To me the first class is worth it, wider more comfortable seats, wrote a trip report that has some photos of the inside of the AP train if you want to check it out scroll to the AP train photos.
https://www.fodors.com/community/europe/extended-trip-to-portugal-1711933/?ispreloading=1

For the trains out of Porto the same rules apply.

Is the Navegante Occasional Card only for use for Sintra and Lisbon and day trips from there?
Are such tickets time/train specific or do they just draw down enough to cover a fare like a London Oyster Card? Are the cards used by a tap on to enter train gate? Must you use it to exit a train? Other than a Navegante Card, can you buy paper tickets or just tap on, tap off with a credit card?

How are tickets purchased for day trips such as Porto-Guiarmes, Porto-Braga and Porto-Coimbra?

Navegante Ocasional card is only valid in most of the Lisbon area transit systems, but not all accept the card.
We loaded money into the card instead of tickets and used the zapping function.
In most stations you'll have to scan the card at the entrance and exit.
On the Urban train, Metro you'll have to use the Navegante Ocasional card, not valid on long distance trains.
Details of the Navegante Ocasional card site.
https://www.metrolisboa.pt/en/buy/viva-viagem-card/

In Porto you'll most likely buy InterCity train tickets, they can be bought online or in person at the train station.
Use the official train site
https://www.cp.pt/passageiros/en/buy-tickets

I assume we can just buy paper tickets from a machine or ticket window for the Urbana Train from Sintra-Lisboa Oriente, correct?
Must you designate a date and/or a specific time for that train when you purchase it or do you simply buy a one way ticket for the Sintra-Lisboa Oriente Train?
Whenever we can we will buy tickets from the Cp.pt. App. for AP and IC train trips.
We will spend four nights in Lisbon at the end of our trip and decide then about the Navagente Card and the use of single ride or daily tickets. I think we will use Uber for many of our travels around the city to get from one area to another.

Can't buy paper tickets for the Urban trains in Lisbon, you must buy the Navegante Ocasional card and load it with either money or tickets.

Urban trains don't have reserved seats , just show up and board the train at any time.

Uber and Bolt are available in the Lisbon area.

“you must buy the Navegante Ocasional card and load it with either money or tickets.”
So the tickets you would load would be for a ride between Sintra-Lisboa Oriente at any time, on any date? When you load it with money and scan it at the train gate, it just draws down the amount needed for the trai fare?
In effect, the Navagente Card is akin to a prepaid debit card for train travel?

Correct, if you load money into the card the money is debited from the card everytime you use it in Lisbon Urban trains, the card can also be used in Lisbon buses, Metro and ferries.

Don't load too much money into the card since it's not refundable.

Can you just load a single Sintra-Lisbon ticket on the card? We will be traveling on to Porto that day on a AP train which we will book online. When we get back to Lisbon about a week later, we will load the card with money or buy 24 hour passes.

Yes, you can load a single ticket into the card.

Thanks for all the help!

Must you choose what type of ticket to load on the card, e.g. single ticket vs. 24 hour pass vs, euro credits ? If you have more than one type , how is the ride fare drawn down?
Also, without a navegante card, can you just tap on with a credit card for a metro or bus ride?

You must choose what you want to load into the Navegante Ocasional card, money or ticket.
You can only use one at a time ( money or ticket), once you use all the money or the ticket you can load a different ticket into the card.
There are different tickets available, you can only load one type of ticket and use it before loading a different type of fare.

ATM contactless cards ( bank card) are accepted in the Metro only. Some tourists report having issues with it as their banks freeze the cards to prevent fraud.

You can view the different types of fares on this site.
https://www.metrolisboa.pt/en/buy/

The buses inside Lisbon ( Carris) accept the Navegante Ocasional card and cash but no bank card.

Urban trains in Lisbon only accept the Navegante Ocasional card loaded with the appropriate fare ( money or ticket).

Or just to make it easy, load some money on the card and call it good. Then you can just tap and go, no reason to worry about single tickets, 24 hours pass, etc. etc. The card will be good for your Sintra train trip and the transportation in Lisbon.
Transportation is so cheap anyways, I wouldn't spend the energy trying to figure out the rest. The 24 hour pass is rarely worth it, cheaper just to load money on the card. And when out of money on the card, just go to a machine and load more. Very easy. Anyways, the cards are good for a year. For Sintra, just tap the card and jump on the next train.

And for Lisbon to Porto, look at Seat 61. I traveled 2nd class and was happy with it. Cheaper to buy in advance.

https://www.seat61.com/trains-and-routes/lisbon-to-porto-by-train.htm

Thank you both for all the help!