Make some stops along the way to visit places that interest you.

If you're not interested in that, there's also a ferry from Barcelona to Civitavecchia (which is close to Rome):

https://benefitsportal.eurail.com/benefits/grimaldi-lines/

Madrid to Barcelona by train, Barcelona to Civitavecchia by boat (interrail discount (20%) available) [https://www.grimaldi-lines.com/en/], Civitavecchia to Roma by train (does cost just 4,60 €, better buy a normal ticket - so you'd need just one travel day).