« Concerns have been raised about intermittent faults when opening the doors of the Class 377 trains at certain stations on the Thameslink route. » The original problem? Engineers decided to use GPS location to determine whether the trains are in a station. Unfortunately, GPS is provided by satellites, so it doesn’t work in tunnels or underground — for example, at stations like St Pancras. The engineers’ solution: Install GPS repeaters in the stations to transmit GPS signals down the tunnels.