Authorities blamed high speed and broken regulations for the crash, Spain's second deadly rail accident in as many months.
Among the train's 426 passengers were foreign pilgrims returning home after visiting the tomb of Saint James in the city of Santiago de Compostela.
The train was travelling to the French border when it came off the tracks on a bend at the town of Villada, about 30km northwest of the regional capital Palencia.
The train's first carriage smashed into the pillar of a bridge, ripping off its side and dragging other carriages off the rails.
"The carriage started to move from right to left, all the windows broke and bags were flying everywhere until it finally stopped," an unnamed passenger told Spanish state radio. "There was a lot of smoke. I thought there had been a fire."
Emergency authorities set up a field hospital in a railway building as four helicopters ferried injured to nearby hospitals.
A spokesman for the Interior Ministry said five people were pronounced dead at the scene while a 30-year-old man died in hospital in the city of Valladolid.
Two other injured people were reported to be in serious condition while the rest were treated at the site of the accident, he added.
Spanish Development Minister Magdalena Alvarez said railway authorities breached regulations by joining carriages of a high-speed train to the back of a regional one to save time after the service fell behind schedule.
In July, 41 people were killed when an underground train ran off the tracks and overturned at high speed in the eastern city of Valencia.