A trolley car slammed into the back of another one that was stopped for a safety test Tuesday morning on the Arakawa tram line that runs through Kita Ward, Tokyo, injuring 25 passengers and one safety inspector, the Tokyo Metropolitan Government said.

Twenty-one people were hospitalized, said an official of the metropolitan government, which operates the line.

Police were trying to determine the cause of the accident, but they suspect negligence on the part of the motorman in the trolley car that was moving. The line has no automatic emergency braking system.