The door must detect a 1-2cm obstruction (e.g. fingers, child's wrist) and be sufficiently weak that a child can pull their hand/wrist free. The door must not prevent a train from departing if only a hem of a skirt/trousers/coat is trapped. They must be strong enough to prevent anyone falling from the train while it is in motion, and must not be able to be forced open between stations.
Otherwise, approximately one person per year will die.
How much would you charge, to make doors to this specification?
And, some time in the next five years, an accident will happen, somewhere in the world. Your next door will need to prevent that situation, too.
[This is the situation in "wealthier" Europe, where it's pretty difficult to die on a train. Other countries may vary.]
If the padding is too squashy, a trapped hand won't be detected and the person could be dragged under the train. Too sensitive, and someone leaning a suitcase on the door prevents the train from leaving.