From Siena to Florence to Milan

As I recall, the bus station is about a block from train station -- in other words, walking distance. If you want to build in possible traffic delays from Siena, how about reserving the one-hour-later Milan train ?
Also, trains occasionally have delays as well.

Laura B is right; the bus station is adjacent to the train station, as I recall.

I'd rather not spend an hour or more waiting in the train station if I can avoid it. It's usually crowded and is not very interesting.

And yes, it's just a short walk from the bus to the train station.

I am not sure I understand the question. The OP poster knows that the bus station is close to the railway station. Ten minutes would be enough to make a connection, five if rushed. The OP is concerned about traffic delays, but looks not willing to add more buffer time. So I wonder what kind of reply he is expecting here.

The recent opening of a bypass tunnel in the Galluzzo area has removed a source of possible traffic delays, the opening of the T2 tramway line has added traffic congestion within sight of the SMN railway station. 15 minutes should be enough to cope with most traffic delays but you cannot rule out one of those catastrophic times when urgent road works or an accident block the traffic.

Also, trains to Milan arrive at the station 9 minutes before their expected departure time. A 5 minutes train delay is fairly common - but you can't be sure - and sometimes also trains are delayed by unexpected events.

If you get to Siena on Trenitalia they will reprotect you on a later train if the incoming train is delayed. If you arrive by bus it is your responsibility to get to the railway station in time.

That is all. But ultimately the decision of taking a slight risk versus spending more time is OP's decision.

Assuming no traffic snarls, 35 min are more than sufficient. The bus station is across the street from the station on the Bar Deanna side (practically next to Bar Deanna).

If you want to be guaranteed that your tickets for the high-speed train won't be invalid should you arrive late in Florence and miss that train, then buy Trenitalia tickets from Siena to Milan (regional to Florence and high-speed to Milan). If the regional train arrives late due to Trenitalia's fault, you can exchange your high-speed train tickets for a later train at no charge. Note that this won't work if you book an Italo high-speed train from Florence to Milan.

Another option, although more expensive than pre-purchasing tickets, is to buy tickets for the next high-speed train to Milan upon arrival in Florence. There are apps you can use that won't require you to go to a ticket window or kiosk.

"I am not sure I understand the question."

I'm trying to get a feel for how likely it is that the bus would be delayed significantly. If there were a very high chance of either it being on time or being delayed significantly (e.g., more than 15 minutes), then a decision becomes easier. Presumably a 40 minute delay in the regional train is highly unlikely and a 23 minute connection time for the Frecce in Bologna is safe?

If the bus works, it would be the fastest way to travel and would minimize changes (one). Bus (1:15), wait (:35) and train (1:50) or a total of 3:30.

The bus to the one change high speed would be bus (1:15), wait (1:10) and train (2:00) or a total of 4:25. Regional to one change high speed would be 1:32 plus :40 plus 2:00 or a total of 4:12, plus travel from Siena center to the train.

Bus plus a changeable train ticket is another possibility, but seats may be scarce at the last minute (based on looking the past few days). Perhaps when we travel in mid-November it will be less crowded.

In spite of living in Florence and making the trip to Milan fairly frequently, I still cannot make up my mind for you, because being absolutely safe and taking the shortest possible path are mutually exclusive choices. 35 minutes is a fairly adequate time under most circumstances, but if something goes wrong the margin is not so large. How likely is a delay? In 30+ years my best travel time between Florence and Milan is 1:53 door to door including urban transfers, the worst was around 9 hours (without considering strikes).

Just consider that if you want to be safer you have to add travel time, or that in the worst case you will have to buy new tickets for a later train in Florence (there are worst places to be stranded than the railway station in Florence).

I'm not asking you to make a decision for me, I'm just trying to see if there's any way to judge the odds of the Siena to Firenze bus making it more or less in time to make the connection with the Milan train. There may well not be any way to judge the on time percentage for either the bus's arrival or the train's departure.

FWIW, I'm thinking of using the bus with a 35 minute connection at a cheap price. if it works, great. If not, we'll buy the more expensive base ticket once we get to Florence (realizing this isn't necessarily the best decision financially).