Juri Ratas of Estonia, Maris Kucinskis of Latvia and Saulius Skvernelis of Lithuania wrote their names on the dotted line in Tallinn.
A release from Kucinskis' office said the project was "In the strategic interests of all three Baltic states and the wider region."
"It should be implemented without delay!" (sic) Kucinskis was quoted as saying in the release.
A long series of agreements in principle and memoranda of understanding have already been signed and filed away in recent years, but Tuesday's agreement is supposed to be the definitive moment at which all three countries fully commit themselves to realize the €5 billion project.
When built, the Baltic states will gain a high-speed European-gauge rail link to the rest of Europe with hopes that journeys from Tallinn, Riga and Kaunas (with a spur to Vilnius) to Warsaw and Berlin will eventually become routine rather than the painful long-haul trials of endurance and ingenuity they are at present.
Such a link used to exist in the form of the famous Nord Express of the 1920s and 1930s, which allowed passengers to take a train in speed and comfort from Riga as far as Paris, Brussels and even London.
According to the European Commission, the project should be completed by 2030, but the agreement signed in Tallinn specifies that the project should be finished by 2025 with the first trains running in 2026.
However, much work still remains to do, with the route not finalised and numerous documents needing to be submitted to obtain the massive EU funding that will pay for 85% of the projected costs.
The parliaments of all three countries also need to ratify the document signed Tuesday.
The joint venture of the three Baltic States “RB Rail AS” registered in Latvia, has founded branches in Lithuania and Estonia, it said January 30.
"The aim of the newly registered branches is to promote successful project implementation operations," the company said.
The founding of the branches in both countries was in accordance to the Agreement on Rail Baltic/Rail Baltica Contracting scheme signed in October, 2016, defining procurement roles of the joint venture “RB Rail AS” and the national implementing bodies in the three Baltic countries.
“RB Rail AS” is a joint venture between the three Baltic States established in October 2014. Holding equal shares in “RB Rail AS” are “Eiropas dzelzceļa līnijas” SIA in Latvia, “Rail Baltica statyba” UAB in Lithuania and “Rail Baltic Estonia” OU in Estonia.