How do train speed controls work?
An investigation is underway in Spain into the country's worst train crash since 1944 - killing at least 78 people.
The train derailed just before arriving in Santiago di Compostella - sending eight carriages crashing into each other.
The rail firm says there were no technical problems with the train and that it had been inspected just hours earlier. It is reported the driver is now under formal investigation.
It is thought the train was going much faster than the speed limit of 80 kilometres an hour.
Jeremy Ackland, a rail expert from the Institute of Engineering and Technology, explains how train speed controls work.
- From the section Europe