Shuttle trains are fixed fleets of wagons that run at regular intervals on pre-defined routes. It makes no difference whether semi-trailers, containers or conventional wagons are being transported. These pre-defined routes allow fast and efficient. In Spain, the term "autopista ferroviaria" refers exclusively to the transport of semi trailers. This requires the use of special wagons: pocket wagons. In addition, the lines on which these "autopista ferroviaria" operate must have a higher loading  gauge (P400 - P420) than the usual gauges for the transport of containers or conventional wagons. In Spain, "autopista ferroviaria" are currently only used on the Madrid - Valencia line. The Zaragoza - Algeciras line is scheduled to be opened to  “autopista ferroviaria” by 2026.