[1][2] The decision to build the line was made in 1912, and the most difficult and expensive of the route options was chosen.

The decision was influenced by the experience gained from the construction of the curved and hilly Haapamäki–Jyväskylä railway, which proved inadequate for the growing train traffic.

The line became straightforward - plans to bypass Leppälahti and Pönttövuori were abandoned.

The main challenges of the construction process were at the western end of the line - namely, the cliffs between Vaajakoski and Leppälahti, the long lake embankments and the tunnel through the Pönttövuori.

The old Riihimäki–Saint Petersburg railway was so close to the coast of the Gulf of Finland that it was feared that it might be captured by the enemy in the event of war.