To travel with your Train+Flight Combo Ticket, you must first obtain your reservation code:
The Commuter Trains ticket can be obtained at the ticket window or the self-service machines in the stations using your train ticket.
How do I obtain my boarding pass and train tickets?
Flight travel: you can obtain your boarding pass at Iberia.com, Serviberia, and Renfe.com.
Train travel: The ticket for the train journey can be obtained at Iberia.com, Serviberia.com, and Renfe.com. You can also obtain it from the self-service machines in the stations on the day you travel.
To board the trains on the C-1 line at the Madrid Hub to travel between the Puerta de Atocha or Chamartín stations and the Aeropuerto T4 station, you must obtain a Commuter Trains ticket at a ticket window or self-service machine by entering the localiser code that appears under the "CombinadoCercanías" heading on your train ticket.