All are correct answers except Center Harbor and smuggling. RG told me some history/legend about the Boulder Lodge property and why the tunnel was built. Without this tunnel the Mt. Major hike would have been lost until the new parking area was built. Thanks to RG it inspired me to find the tunnel, she found it first by the way, and to see who else here knew about it.
I had taken the original trail before the new Rt. 11 was built and after visiting it the other day I remembered I had taken the tunnel a few times too. I also had climbed over the area after the trees were removed and before the tunnel was put in.
The original trail off Rt. 11D is now a private property. There is a newer trail to the tunnel from 11D which is used by hikers and snowmobiler's.
I believe the relationship between the Boulder Lodge and the RR was very close. I had a conversation a few weeks ago with the new owner of Boulder Lodge and they had heard that the train station was right near their property if not on it. Upon reviewing the photo I posted of the lodge and my own visit

to the property plus McD's post card above I believe they are very closely related. The far left shore of the photo I posted and the small point seen in McD's post card are indeed the same location.
Maybe other members of the forum will have additional info on the trail and the RR station. I know the current owners of Boulder Lodge (now a private residence) would love to know all they can about the history of Boulder Lodge, the tunnel story and the RR station.