There is no prospect of a rail siding at SEN simply because there is not, and never will be, a requirement for such a facility. Even if the proposed runway extension comes about (and that is a big 'if' still) there is no prospect of mainline freighters being able to operate into SEN. All that is likely to result is growth in the express package/urgent freight traffic using aircraft up to ATP size.
It is worth bearing in mind that Ford had planned last Spring, before the motor industry woes began, a twice daily freighter service to Romania following their purchase of the Craiovia car company.
As far as the station becoming a reality is concerned, we obviously won't have long to wait to see if it goes ahead. I'm assured that it will, but I've been involved with SEN for over 45 years, so ...........