Just in front of Restauradores station (off Rossio) where you get the trains to Sintra you will find the Cafe Beira Mar. It is technically on the corner of Praça D. João da Camara and Rua Primeiro de Dezembro.
If it's full, don't be intimidated. They do meals, snacks and will probably pack up sandwiches, local savory snacks and pastries for you.
I've snacked there many many times.
I've snacked on of Portuguese Trains before but remember that the Lisbon to Sintra Service is a urban service and not one is going to love you if you start laying out a full meal while the train is packed.
It's only a +-45 minute run however.
