We're staying in Varenna four nights (late September/early October) and want to "do" the Bernina rail route while we're there. Looking at the current schedule for our possible dates, we'd take the BEX from Tirano to St. Moritz, and a regional train back to Tirano - we really don't want to get up early enough to take a regional train from Tirano to St. Moritz (we'd have to take a train from Varenna-Esino at 7:24 am to get the morning BEX from Tirano).

My ultimate question is how far in advance do we HAVE to book the BEX tickets/reservation? I could do it now but would rather have a better idea of the weather. Making reservations a day or two ahead of travel should give me a better idea of weather than a month out.

TIA for any help you can give me.

Well I think many BEX trains are full with tour groups but why not now just try a dummy booking for a day or two ahead now and see what's up.

I tried that and there were quite a few seats available for next Tuesday. I wonder if they add extra carriages during the summer months?

It may depend on whether or not you think you want to be in one of the Panorama cars. And yes they very much ARE "worth the money."

IME they may also attach local travel wagons to BEX trains - no commentary, etc but like regional trains no reservations.

The commentary on the BEX is not outstanding. It is a "on your left you see..." sort of thing. There is no food service only a mini bar that circulates in the train and sells drinks and souvenirs. The BEX carriages are now attached to normal regional trains. ALL that to say that if you simply took a regional train Tirano/St Moritz - they run hourly - you would do almost as well as taking the BEX itself and without the seat reservation fee.

That said - in summer there are several Bernina Express trains per day and now it is September and the "rush" of July and August is over you have a good chance of just being able to get a seat reservation on the BEX the night beforehand. My suggestion would be to take the BEX Tirano to St Moritz and then go back on a normal train OR better yet if possible spend a night in St Moritz or the environs and take the bus via the Val Bregaglia (stunning scenery but very different from the Bernina pass) to Chiavenna and then train from there to Varenna. Those buses to Chiavenna run once or even twice an hour - you can check on www.sbb.ch/en

The "commentary" which was mentioned above is a great deal more than "on the left" and it is now done via personal devices such as one's cellphone.

You cannot lose.

If it's not full, which it probably won't be, you'll get reservations in the panoramic cars even on the day.

In the unlikely event the panorama cars ARE full, you just sit in the Allegra unit up front in the unreserved seats, which cannot sell out.

Allegra units have commentary too I assume?

If that is the case it has improved since last summer! I did not take it this summer...

I suppose you know that regional trains between Tirano and St. Moritz run about every hr:
Tirano dp 7.41, 9.00, 9.41, 11.00, 11.41, 13.00, 13.41, 15.00, 15.41, 17.00, 17.41
Trains between Varenna and Tirano run every 1-2 hrs.
St. Moritz - Chiavenna - Varenna by bus and train lasts 3 hrs 25 min. St. Moritz dp 7.15, 9.15, 11.15, 13.15, 15.15, 17.15

I'd still advocate for not going all way to St Moritz but aborting trip mid way and doing an easy walk to the glacier. Or if going to St Moritz taking the scenic bus down the Maloja Pass to Chiavenna.

The St. Moritz, Chiavenna, Varenna bus option looks beautiful (just checked out some youtube videos). BUT - I have a bit of a problem with heights/acrophobia, and my wife can be extremely prone to motion-sickness. Even though the drivers may be perfectly capable and manage the hairpin turns like a pro, the only way we could manage this trip would be if we were knocked out with meclazine/dramamine - then what would be the point.
Thanks for the suggestion though!

Yes taken that any many mountain postal buses in Switzerland and yes lots of sharp hairpin turns at not such a slow speed. The train though is smooth and slow. Called the world's slowest express train along with Glacier Express.

The bus goes down the Maloja pass. Which has about 13 haripin (and somewhat steep) bends. Given your combined "problems" this is probably not a route to take! That said - I have acrophobia and the pass didn't both me. I am quite sure it would bother someone with motion sickness though.