The big thing to remember about traveling on any train in Spain is - do not throw out your train tickets or and receipts, until you are well away from your destination train station.
The Cercanias trains are the local commuter trains that serve major cities such as Madrid, Barcelona,Valencia and Zaragoza. As a visitor you might find your self using a Cercanias train to get to a tourist destination a few miles away from a major city. For example, if you wanted to visit the El Escorial you can take a Cercanias train out of Madrid, total travel time about one hour and five minutes.
Cercanias trains do not have seat assignments.
Cercanias train tickets can be bought from machines at the train station and if available, "manned" ticket windows. The Cercanias train tickets are rather small and contain a magnetic strip which can be read by machines at the various train stations. Unusually, but not necessarily always, before boarding your train you will be required to insert the Cercanias ticket in to a card reader to release a turnstile (baffle gate). Sometimes, you might find no turnstile., just a card reader, follow the example of your fellow passengers and use the card reader immediately before heading to your train platform. On arriving at your destination you may need your ticket again in order to exit the train platform area.
The first time I used a Cercanias train ticket (1999) I arrived at my destination and there was no need to show my train ticket again. On the return trip I arrived in Madrid and found myself facing turnstiles, passengers getting off trains were using their train tickets to get through the turnstiles. I had gotten a little too relax on the train; I frantically started to search my pockets looking for my ticket. Lucky for me, an attendant noticed my frustration and wave me through the handicap lane. Since then I have always kept my Cercanias ticket handy on exiting a train. P. S. I have been told, train station attendants these days are not being as helpful to people who have misplaced their ticket.
Tip: plan to use an AVE train but need to use a Cercanias train to connect to the AVE train station? Depending on the city you may be eleagable for free Cercanias tickets, you can read more about it at RENFE.
Next: How to read an AVE train Ticket