Four people were injured when a rail replacement bus was in collision with another bus in a station car park.
Govia Thameslink said it "appeared to go out of control" at Lingfield Station in Surrey at 18:25 GMT on Monday before hitting the back of the stationary bus.
The moving bus then landed on two unoccupied parked cars.
A total of 11 passengers were on the two rail replacement services, with the majority on the stationary bus, Govia Thameslink added.
British Transport Police (BTP) said paramedics treated three people at the scene and one person was taken to hospital "as a precaution". Their injuries were thought to be minor.
Neither bus driver was hurt.
BTP said both drivers "blew negative" on a breathalyser test and inquiries were ongoing.
The spokesperson for Govia Thameslink said: "At around 18:25 a rail replacement bus appeared to go out of control while turning in Lingfield station car park.
"It struck the rear of a stationary bus. The front of the moving bus then lifted and landed on two parked cars, which were not occupied.
"A third parked and unoccupied car was damaged indirectly."
No rail services were cancelled but six trains had to be turned one or two stops before Lingfield, at Hurst Green or Oxted.
Southern's customer services director Chris Fowler said they were "investigating the cause of this serious incident" and encouraged passengers to claim for any delays more than 15 minutes caused to their journeys.