I am considering the 24 HR card which includes train to Sintra from Lisbon. Do I just use that card for the train, or do I use it to acquire a train ticket at the station?
You only need to use the Navegante card loaded with the 24 hour pass to take the train. Are you planning on using the pass for other travel that same day? If it's just for the roundtrip journey between Lisbon and Sintra, it would be cheaper to buy the train tickets and not the 24 hour pass. Either way, you'll need the Navegante card, which is reloadable. At the train station, you can use the vending machines make your purchases.
Thanks for the info. That's a good point. If I spend most of the day in Sintra, I probably won't be using it enough otherwise.