Sadly it is even more complex. While in The Netherlands we are infact actively micromapping stations. It seems that some thing like not connecting the stairs, elevator or escalator to a node connected to the platform results in routing problems such islands in the data. In this case a user is unable to board a service.
Our GTFS feed is really specific, every platform has its own coordinates, so things really would require some automatic validation.
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Met vriendelijke groet,
Stefan de Konink