whilst the daily mail might be given to hyperbole and sensationalist reporting (indeed that's the case here too), I would advise you check out their report, if only for a number of the photos, which are quite clear.
this was an old train (built in the 1980s) and the door in question is in the centre coach, and is where the guard's compartment used to be (these trains are driver only) and for that reason is equipped with a droplight similar to those that were common on most trains with slam doors. in the photos, the attention of the emergency services is on the door on the platform side of the train, which means the victim wasn't struck by a train, rather he came into contact with something lineside (i've had that confirmed). the clearance between trains and (for example) bridges isn't very big, and it wouldn't be necessary to lean very far out to sustain a life ending injury.
the sad fact is that this was completely unnecessary - new trains to replace these old stagers have been available for some time, but the unions have blacked them as part of their dispute with train operator Govia-Thameslink, Gatwick Express's parent.