Train/bus to Siena

I know there are frequent buses between Florence and Siena. These buses are cheap to ride. There is a train station in Siena. I believe you may have to take a cab to the historic town area. If busing from Florence, the bus leaves you a short walk to the Il Campo area. You are on the perimeter of the historic area. It will look like a modern residential area.

I agree: taking the bus spares you getting from the Siena railway station to the city centre.
The bus leaves Florence from the bus station on a side street just across the road from the central station (SMN) and arrives in Siena at the Piazza Gramsci.

There is a bus from the train station in Siena to Piazza Gramschi, which is where the intercity bus would drop you anyway. Sometimes train schedules work out better than buses.

"We have a single country train pass in Italy. Is the bus to Siena included in pass? "

No, as the bus and the train are run by different companies.

"Is it pretty simple to get to bus station/stop from train station? "

That depends on which train station and bus station you are talking about. Most people visit Siena from Florence; in Florence, the two stations are close to each other (a few minutes walk). As you've been told, in Siena there is some distance between them. If you need information about stations in other cities, let us know which ones.

If you are arriving by train in Siena then yes the bus stop is directly across the street from the the train station. Also there is a Tourist Information office in the train station where you can buy bus tickets. We were told all bus lines that stop there will take you the historical center but not necessarily everyone goes to Pizza Gramsci but they do stop in the center of town.