The Dolomites aren't a single place โ they're a mountain range spanning three provinces. Cortina, Val Gardena, Val di Fassa each have different access points. The fast train gets you to gateway cities. After that, mountain roads.
Three gateways: Bolzano (Val Gardena, Alpe di Siusi), Trento (Val di Fassa), Venice/Mestre (Cortina). Train to one, then bus or car.
Rome โ Bolzano: Frecciarossa to Verona (3h15, from โฌ19), regional to Bolzano (1h20, โฌ9). From Bolzano, SAD buses reach Val Gardena in 40 min. Rome โ Cortina: Train to Venice (3h45), then Cortina Express bus (2h, โฌ18).
A1 north to Bologna, A22 Brenner to Bolzano. A car is essentially required in the Dolomites unless staying in a single resort. The A22 is one of Europe's most scenic motorways.
Train to Verona or Bolzano, rent a car there. Don't rent in Rome โ rent at your gateway city and save 5 hours of driving.