Death in the Tunnel

On a dark November evening Sir Wilfred Saxonby is travelling alone in the 5 o'clock train from Cannon Street in a locked compartment. The train slows and stops inside a tunnel; and by the time it emerges again minutes later Sir Wilfred has been shot dead his heart pierced by a single bullet. Suicide seems to be the verdict even though no motive can be found but Inspector Arnold of Scotland Yard thinks again when he learns that a mysterious red light in the tunnel caused the train to slow down.