Not the horror film, but a 1930s melodrama which opens with thirteen passengers on a London bus being involved in a horrific crash; one is killed. The lives of these people and how they came to be on the bus are shown in flashback, and it is only at the end of the film that it's revealed which of them has died.
London General STL24 (JJ4332) is the bus star of the film, although we never actually see its registration plate.
Much of the action takes place on the lower deck. Colin Read notes the scenes look authentic, but could be studio mock-up. Certainly the views through the windows are back-projection, but the footage looks genuine, unlike the
scenes shot in Hitchcock's Sabotage from three years later.
Scenes at the bus garage include outdoor footage of STL24. The bus carries garage code CL for Clay Hall garage, which was then new, and passed to the London Passenger Transport Board during 1933.
There's also a brief street sequence showing a Leyland Titan TD1 of Enterprise on route 14B, followed by a London General AEC NS: