A system which can easily and cheaply be installed to an existing rail infrastructure is disclosed which seeks to avoid the likelihood of train collisions. Each train travelling on a network is provided with a means for determining its own position (such as a GPS receiver) and a means for communicating with a central controller. The train's position is communicated to the central controller and the central controller uses this information to work out where each train in located on the network. Velocity and acceleration information are obtained by monitoring the train's position over time and predictions are made as to the future position of each of the trains on the network. If the future position of any of the trains is determined to be in a range which at least partially overlaps a range of future positions for another train then a collision warning is sent to one or both trains. This warning could trigger the train to stop automatically or for the driver to apply the breaks. Successive restart signals are then sent so that the trains may continue on their journey without likelihood of collision.