Rail Baltica envisages the construction of a 1435-mm-gauge corridor linking Poland with Lithuania, Latvia and Estonia, and potentially Finland via a 60-km tunnel between Tallinn and Helsinki.

The first section to be completed is a 1435-mm-gauge railway line from the Polish border to Kaunas, which has been built alongside the existing 1520-mm-gauge line from the former break of gauge at Šeštokai which has also been upgraded. The project is designated Rail Baltica I, with Rail Baltica II referring to the proposed 728-km double-track electrified line on a new alignment from Kaunas to Tallinn.

A 26-km single-track, 1435-mm-gauge railway line from the Polish border to Mockai and Šeštokai has been modernised, with a 7-km section of interlaced four-rail mixed gauge track laid between Mockai and Šeštokai. The 93 km of 1520-mm-gauge line from Šeštokai to Kaunas has been completely renewed, and a parallel 1435-mm-gauge track constructed with adjacent platforms at Marijampolė, Kazlų Rūda and Kaunas stations. The approach to Kaunas is on a bridge, where the double-track 1520-mm-gauge line has been converted to parallel broad and standard gauge single-track routes. The new and modernised lines are suitable for passenger trains running at a speed of 120 km/h and freight trains at up to 80 km/h.

PKP IC’s Hańcza passenger service from Warszawa and Białystok is to be extended into Lithuania from December-2015 timetable change, although current plans could initially require passengers to change to a LG broad gauge service at Mockai.

Traffic on the new line is predicted to reach 5 million passengers and 13 million tons of freight in 2030.