196

It's really, really simple. Just counter-intuitive.

Each door at the start has a 33.33% chance - and obviously this doesn't ever change.

What changes is that one door opens without your impact, and that door always has people laying on track. Which means the choice is now between two: your initial doors (doesn't change, so 33.33%), and NOT your initial door (100% - 33.33% = 66.66%).

And no, this is not a loophole. The chance is impacted because one door (the automatically opened one) was opened by someone who knew what was behind all doors.