Dirigibles

The Great Sky Line was made possible by one technology alone; the great airships that power through the skies of Alvarica. Though theorised for years they did not enter widespread use until, powered by the industrial might of Clockworks, extraction of Helium on an industrial scale became feasible in 4770. The first fully rigid airship launched in 4764, pioneered by the TransEther Corporation of Hiru.
Dirigibles proved extremely popular, as they could cross national borders without ever touching down, avoiding the logistical nightmares of constructing a continental railroad through dozens of sovereign territories. The first cargo run from Ma'trine to Hiru was established in 4772, and passenger dirigibles began making the trip regularly a year later.