The crash happened in Granges-pres-Marnand in Vaud canton, about 30 miles south-west of the capital, Bern.
Reports say the regional trains collided head-on in the early evening.
The driver of one of the trains was still unaccounted for and thought to be inside the wreckage, police spokesman Jean-Christophe Sauterel said.
“These are regional trains. The speeds are a little lower and even if one deeply regrets the likely loss of life of one person as well as five serious injuries, the situation could have been much more catastrophic,” he said.
Jean-Christophe Sauterel said it was too early to determine the cause of the crash.
One of the injured was taken by helicopter to Lausanne. Others were treated at the scene by paramedics or taken to hospital by ambulance.
Earlier this month, 79 people were killed in a high-speed derailment in north-western Spain.
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