Authorities could confirm 10 deaths, Castilla La Mancha region spokeswoman Esther Padilla said, but that "that number will surely rise".
It was the second major train train accident in Spain in 2026.
Rail system operator ADIF said the crash occurred on Wednesday morning in the province of Toledo.
Rail service was suspended between Madrid, Ciudad Real and Andalusia.
Padilla told state broadcaster TVE that the accident was between two high-speed trains heading in the same direction from Madrid to Seville.
One ran into the tail of the other that was stopped, knocking two of its cars off the track.
She said the cause of the crash was not known.
Spanish Health Minister Mónica Garcia told TVE that emergency workers were focusing on rescuing survivors and bodies from the two most damaged train cars.
She said the injured were being sent to different hospitals in the area.
"It was a high-speed impact, so we are expecting this to be a tragedy," García said.
The accident involved a train from Spain's state-owned RENFE company and the private rail company IRYO.
It appeared the RENFE train rammed the IRYO train, Spanish media reported.
Trains from those two companies were also involved in a deadly crash in January.
Padilla said that 397 passengers were in the RENFE train and 147 passengers in the IRYO train.
IRYO, an Italian-owned company, said in a statement that it has "activated its security protocols and is closely working with authorities."
The mayor of the nearby town of Urda, Manuel Galán, told Spanish state broadcaster TVE that passengers had been injured and that civil protection authorities had asked his town hall to prepare a sports pavilion to attend to passengers.
The regional president of Castilla La Mancha, Emiliano Garcia-Page, wrote on X that he is on his way to the crash site.
In January, 45 people died in southern Spain when a train jumped the track and hit an oncoming train.
The crash was the deadliest in Spain since a 2013 crash that killed 80 people.
The January incident shook the confidence the European country has in its extensive high-speed rail network.
with AP