We will be in Chamonix September 30-October 2. I can see that some cable cars/trains are already closed for the summer season. Which ticket do I pick? A two day or should I just be happy with getting to go to AdM?
From the looks of it, everything except AdM will be closed by the time you get there.
See footnote at the bottom of referenced page.
Chamonix lifts
A 2 or 3 day multipass costs very little more than a 1 day pass, and you have the option of going
up multiple times, if that is of interest to you. A single ticket for the AdM is 81 EUR, the 2 day pass
is around 100 and the 3 day pass is ~125 (depends a bit on age).
So if you want to go up only once, just get a single ticket. If you think you might want to go up
more than once, buy a 2 or 3 day pass. This would also be the most efficient way to guard
against weather suddenly coming in and making your initial ascent ~useless (when I went up
in August, it was snowing at the top. 2 days later, sunny.)