A spokesman for the Vulkan-Eifel-Bahn company (VEB) said those who suffered serious injuries included the truck driver, the train driver and a conductor. They were airlifted by helicopters and flown to a nearby hospital.
The spokesman added that their conditions had stabilized by Sunday.
The incident took place in the region of Hohenfels-Essingen in the Vulkaneifel district in the southwestern German state.
The cause of the accident is still unclear but the VEB spokesman said the truck driver probably overlooked the warning signs near the unmanned crossing and didn't see the train.
Witnesses said the historic diesel-run train slammed into the truck's trailer. The collision lifted the train from the tracks and sent it crashing down a three-metre high embankment. The train's nose was reportedly completely ripped apart by the force of the collision.
Police have sealed off the tracks and are working to tow away the wreckage.