Travel from Siena to Assisi

I'm not going to be able to give you complete information but I can urge you VERY strongly NOT to miss Assisi! It is an amazing experience, not just a beautiful city on the side of a mountain.
I've done this route in the reverse and it isn't awful: Siena to Florence by bus, Florence to Assisi by train. I understand that the Freccia line is a possibility but honestly, the travel time is not much less than the local train but it very much more expensive. And there is not frequent service. Hmm, where would you start this trip? Would it be easier, and more time efficient, to do Assisi, Siena, Orvieto? Or the reverse?

You could take train or bus to Florence
Then from Florence train to Assisi

From Assisi to Orvieto take train to Perugia, change to train to Orvieto

https://www.trenitalia.com/content/tcom/en.html

All regional trains-can’t sell out and price never changes

Thank you for the information, I will look into both options.

I would go by bus via Perugia, because to go by train via Florence you are looking at a 4 hour trip. By bus, if all goes well with the connections, you can get to Assisi in 2.5 hours.

Two companies operate the route Siena-Perugia
Flixbus (2 buses, depart at 9:30 and 14:35, the latter proceeds to Santa Maria degli Angeli, which is Assisi's train station, so no changes necessary, except for the bus or taxi from the Assisi station to the actual town)
Autolinee Federico (1 bus departs at 17:30, so might get there a bit too late for comfort).

Unless you take the Flixbus at 14:35 the others would require a change at Perugia's bus station. From the bus terminal on Piazza Partigiani in Perugia you could take Bus E007 operated by Busitalia, which will take you directly to Assisi (hill town) rather than Assisi train station, actually located down the hill at Santa Maria degli Angeli (the town that gave the name to Los Angeles in California)

Thank you Roberto. I am researching those bus options.