Faced with the major increase in demand in large commuter cities, Renfe decided to acquire double decker stock in order to offer the best solution for long commuter lines with route times over 30 minutes. Twelve sets were initially put into circulation comprising a trailer with cab and four intermediate carriages towed by 269-200 series engines, which were later replaced by double decker engine cars. The 450 and 451 series trains changed to the current composition in 1994: two engine cars and four intermediate trailers for the first series and one engine car, one trailer carriage and one trailer carriage with driver's cab for the 451.
Each trailer carriage features 100 seats on the lower floor and 96 on the top floor. In turn, each engine car has 50 seats. The seats form modules separated by a central aisle consisting of four and six seats facing one another in two- and three-seat rows. Each carriage has a toilet on the lower floor and all are equipped with air conditioning, ambient music and indoor displays, informing passengers of the final destination of the train, the next stop, the time and the outside temperature, providing visual and acoustic information en route.
It boasts an air conditioning system. Heating is reinforced with floor-level heaters in the compartments.