At least 47 people were injured when two packed light-rail trains collided at a boarding platform on San Francisco's west side in US, nearly a month after seven people were killed when a metro slammed into another in Washington.

Both trains were full when the crash occurred yesterday afternoon near the West Portal shopping district. Four people were severely injured although no deaths had been reported so far, San Francisco Chronicle reported. Authorities said a San Francisco Municipal train rear-ended another at 2.30 pm (local time). Both trains were operated by the San Francisco Municipal Railway, or Muni, system, the paper said. "This is probably one of the largest (accident) we've had recent in years," Deputy Fire Chief Pat Gardner told the paper adding that about 20 people suffered "moderate injuries".

Witnesses who described the scene as chaotic said the westbound train had just emerged from a tunnel that connects San Francisco with its western neighbourhoods when it struck the second train. After the collision, more than a dozen passengers mostly adults sat on platform benches holding bloodied heads while 40 rescue workers set up a triage system to isolate the most severely injured. The driver of the first train was among the four who were injured seriously.