For scenery and railway engineering: Grožnjan to Livade.
If you have limited time, the official Parenzana project itself describes the Grožnjan–Livade section as the most attractive part of the route because it combines six tunnels, four viaducts, bridges and wide views. That is a far stronger reason to start here than an arbitrary kilometre count.
Why it works
You begin near Grožnjan, one of Istria’s strongest hill-town stops, then follow the former railway through Kalcini and Kostanjica tunnels, the two Završje tunnels, and major stone structures including the Završje, Antonci, Freski and Oprtalj viaducts. The route then falls toward Livade and the Mirna valley.
Add Motovun rather than rushing
The best experience is not necessarily the purest “railway-only” ride. If time allows, continue into the Motovun landscape or use Livade as the transition into a stay around Motovun. The hill town is above the former line, so visiting it means accepting extra climbing or planning transport.
Who should choose another section?
Families and road-oriented cyclists may prefer smoother Slovenian sections. Stronger riders who want a full-day physical challenge can use the 80 km Trail 19 as the main objective. Riders focused on architecture and photos gain more by slowing down through the heritage core than by maximising distance.