Carry On Spying Trivia
Mr Simpkins hands the formula to Leila in his railway compartment. The train brakes hard which gives him the opportunity to take formula and revolver from her. A few moments later he leaves the compartment to the outside of the carriage. The outside shot indicates that the train is moving - not stationary after braking - which can also be inferred from the noise that can be heard when the wheels run over the joints in the rails.