Only 9 years too late

When my sister and I were 9 and 13 respectively, my Dad drove us to Welshpool train station. We were accompanying our cousins who were taking the train home after a visit to our house. The scenario was as follows: the train arrived, my Dad got on with the cousins to help with baggage, my sister and I were still on the platform.

The train then leaves. Dad was still on the train. So we decided to go to the station sweet shop but our plan was thwarted; we had been spotted by a policeman who took us to the nearby station to wait for Dad returning from the next town in a taxi.