The more I read on this the more I'm confused. This is from the Single tourism website:
Single tickets for both Rossio-Sintra and Oriente-Sintra cost €2.30, and can be bought at the ticket counters or machines. The tickets are magnetic cards called “Viva Viagem,” which you need to purchase the first time you take the train in Lisbon. In future journeys you just have to recharge it. It costs €0.50, so if your trip to Sintra is the first time you’re taking the train in Lisbon, the total cost will be €2.80, and for your return to Lisbon you just need to add a single trip ticket to the card (for €2.30). There are no return tickets, so to go and return you’ll have to buy two single tickets.
It’s important to note that if you previously acquired a “Viva Viagem” card to use on the metro or trams, it can only be used for the train if it has no fares left. Your balance has to be zero, otherwise you must buy a new card for the train. The card is valid for just one person, meaning everyone in your group will need their own.
It wasn't clear in other reading that you couldn't use the same card. I'd planned to get one and add enough on it to cover planned bus, tram and metro rides plus the train. Guess not.