Estoril is the second last station on the Linha da Cascais suburban railway that runs from Cais de Sodre in downtown Lisbon to the town/suburb Cascais.
Trains run between every 15 minutes to every 30 minutes depending on the day/time of week/day/night.
Trains start at 05:30 and run till 01:30.
Note there are some trains that terminate at Oeiras half way to Cascais. Make sure you get on the one for Cascais.
