There's no train connection with Riyadh Airport.
Car and bus
Riyadh Airport is situated 35 kilometers north of the city center. The airport is easily accessible by rental car. From the city you follow the King Fahd Road or Al-Takhassousi Road. The airport can also be reached via the East Interstate Highway. The way to the airport is clearly signposted along all these roads.
Apart from the shuttle buses of the large, international hotels there are no bus connections between the airport and Riyadh.
Car rental Riyadh Airport
You can find counters of car rental companies in the Arrivals Hall. Two international rental companies (Avis and Budget) and several local ones have a presence. Only men can rent a car as women aren't allowed to drive a car in Saudi Arabia. You can also book a rental car in advance using the service provided by EasyTerra; they compare the rates of several providers in a single quote.
Taxis are the only means of public transport between the airport and Riyadh. At the airport you'll only find officially registered taxis (recognizable by the airport logo), for which you need to pay upfront at a ticket kiosk. The ride to Riyadh's center takes about half an hour.
On both sides of the mosque opposite the terminal there's a three-story parking garage. Both garages, with a total capacity of more than 11,000 cars, are connected with the terminal.