Hate to burst anyone bubble, but please read below.
I use to be a member of the IRM. If they take it out again on a Mainline Excursion it will fill VERY quickly. I have not been a member for 4 years now, but the last time it went out I am aware of was about 8 years ago. Once Working Members were given a chance, then tickets were offered to associate members. I did not go because it was on a weekday and couldn't take the day off from work. I have ridden it many times at the museum. The Parlor OBS is where you want to be. But unless you are a working Member you are just dreaming about riding there on a Mainline excursion.