Train horn

I must admit I was surprised to learn even DLR trains have horns. We were on a train to Woolwich Arsenal and stopped at it was either Limehouse or Westferry and the train was only two carriages compared to the previous three carriage train to Lewisham. As a result we stopped a way from the end of the platform and a person was standing at the end looking confused. The train operator pressed a button on the control panel which sounded the horn resulting in the passenger sprinting for the train.