There are always going to be circumstances where doors close on passengers - perhaps the passenger is deaf, or a child jumps in/out just as the doors are closing, or someone realises they are about to miss their stop. The railway provides two safeguards to prevent this becoming a more serious incident, and both failed on the day:
- The door interlock should not be made if there is an arm trapped, though it might be if a smaller object such as an item of clothing or a toddler's harness is. A gap wide enough for an arm is probably too wide to be accounted for by flexing of the edge strips, suggesting that the plug door mechanism wasn't fully home. If so the integrity of the door itself has to be open to question and had this incident not brought it to light it is possible the door might have later failed to close properly and subsequently opened at high speed.
- Whoever is doing the train safety check, apparently the guard in this case, needs to confirm not that door interlock has been obtained (so ignore the orange lights!) but that the doors have closed without trapping anything. If the platform is a concave curve this may involve taking several steps from the train so as to be able to see all of it, and if for some reason this is impossible or causes too much delay then platform staff should be provided.
There are probably many occasions where one or other of these safeguards has failed but in this instance they both did. Fortunately here there were other people around to operate the door release and alert the conductor - but these can't be relied on, for example on an early morning or late evening train with few people around. It's also possible that the person might have been able to pull her hand free before the train got too fast to run alongside or reached the end of the platform.