Trenitalia operates a train from Siena to Bologna hourly. Tickets cost $7 - $25 and the journey takes 1 h 32 min. Trenitalia Frecce also services this route every 10 minutes. Alternatively, Baltour operates a bus from Siena to Bologna 4 times a day. Tickets cost $10 - $35 and the journey takes 2 h 22 min. FlixBus also services this route 4 times a day.
From Siena train to Firenze S.M.N. then train to Bologna Centrale. 2 h 13 min 25€. From Siena bus to Bologna. 2 h 22 min 14€. From Siena-Via Tozzi bus to Firenze Santa Maria Novella Station then train to Bologna Centrale Station. 2 h 26 min 24€. From Siena drive to Bologna. 1 h 52 min 25€. From Siena rideshare to Bologna. 2 h 32 min 9€.