Now there is no reason to get on the train without validating your ticket or pass. Buy your Oporto urban train tickets in advance from a CP ticket office, ticket vending machine or from any of the 600 or so Pagaqui agents, and if you buy 10 journeys to top up your Siga card (full-price tickets), you get another journey for free.
Always remember to validate your ticket before boarding the train. “Tickets cannot be sold on board the trains unless there is nowhere to buy one within 200 metres of where you got on” (General Transport Conditions, chapter III.5.1.).
Oporto urban train tickets and passes have to be validated before you can board your train. If you get on without validating your ticket or pass you can be fined between €120 and €350 - Law 28/2006 of 4 July.
Thank you for your cooperation.
Further details here