Shortly after 05:00 local time (04:00 BST) on Monday, a double-decker train set off from the coastal Finnish city of Oulu, rumbling its way across the border to Sweden.
As the inaugural service crossed the Torne river from Finland and pulled into Sweden's Haparanda station, it was met by crowds of people waving Finnish and Swedish flags.
The route was initially suspended in the late 1980s to the early 1990s, due to low passenger numbers and technical differences in the tracks between Finland and Europe.
Finland uses the same track widths as Russia, which are wider than the European standard - a holdover from when the Nordic nation was part of the Russian Grand Duchy in the 19th and early 20th centuries.
It is currently in the process of updating its track widths to match the European standard.
The new, twice-daily journey to Sweden's Haparanda takes 100-minutes as it hugs the west coast of Finland before crossing the border between the two Nordic neighbours.
Passengers travelling on from Haparanda will need to transfer onto a different train to continue southward.
But the service makes it possible to travel around 5,000km by rail from Portugal's Algarve to Finland's northernmost train station of Kolari - believed to be the longest train route in Europe.
It will also enable a 24-hour train journey between capitals Helsinki and Stockholm.
Finland's transport minister said the return of the cross-border service shows cooperation between the two countries was growing "all the time".
