"Currently, the Baltic states operate Russian-gauge railroad tracks, while other European NATO members utilize a standard European gauge (a single line from Poland to the Lithuanian city of Kaunas is the sole exception). This incompatibility means that trains carrying military equipment and supplies from larger NATO bases in Germany or Poland would have to transfer their cargo to Russian-gauge trains or proceed via ground convoys to their destinations.
Baltic rail infrastructure significantly lags behind other European nation-states. A north-south axis across the three countries is currently nonexistent. Plans to correct this are already in motion in the form of Rail Baltica, the largest EU infrastructure investment project in the Baltic states. Ultimately connecting the capitals of Estonia, Latvia, and Lithuania via European-gauge track (along with an additional connection to Helsinki via Tallinn), Rail Baltica would allow for a standardized and uninterrupted rail link to the rest of NATO, with freight service up to 120 kilometers per hour. Though broad commitments by the EU and Baltic states have been made, the project is not expected to be completed until 2025.
One potential source of conflict is Kaliningrad, or more specifically, the railway linking Kaliningrad to mainland Russia via Lithuanian territory. Kaliningrad transit was a political issue back in 2003 when Lithuania was preparing to enter EU. Moscow and Vilnius then reached an agreement that would allow Russian citizens to travel to and from Kaliningrad with a “facilitated transit document” instead of a visa. This document is easier to acquire than a visa, as it requires only twenty-four hours’ notice and far less scrutiny.
As a 2015 report from Center for European Policy Analysis argues, this might be a way to transport irregular militias to Lithuanian territory—similar to the “little green men” in Crimea or the Donbass. In August 2014 a Kaliningrad transit train halted midway near Kaunas, Lithuania’s second-largest city, and its hydroelectric power station. Seeking to prevent any unauthorized exits, the authorities scrambled to establish a perimeter around the train and to identify and address the causes of the interruption. Although the reason for the train’s halt ultimately turned out to be mechanical, the response to the incident underscores the concern that authorities have held regarding any irregularities.
Russia could also exploit Kaliningrad transit by staging a sabotage or “terrorist” attack on a railway, accusing Lithuania of deliberately failing to deal with the issue, and then pursuing an armed invasion masquerading as a humanitarian mission, thus cutting off Lithuania from Poland. Such scenarios may seem alarmist but are not without precedent: in 2014, the Russian government pushed a convoy of over one hundred trucks (ostensibly carrying humanitarian aid) into eastern Ukraine without Ukrainian or international permission.
In 2015, former CEO of Latvian Railways Ugis Magonis was apprehended by Latvian anti-corruption officials in a too-cliché-for-cinema scene involving a high-speed car chase and half a million euros in a trunk. Magonis was himself close to the head of Russian Railways, Vladimir Yakunin. (Yakunin was placed on a US blacklist after the 2014 annexation of Crimea, although he avoided EU blacklist due to Latvian lobbying.) Russian Railways has also increased its holdings over the Latvian rail carrier LNT. The deal itself was not transparent and reportedly involved multiple offshore dealings. All these cases indicate that the Baltic railway system may remain vulnerable to Russian influence.
The ability for Russian forces to rapidly airlift men and materiel within its borders (and its “near abroad”), the increasing force projection capabilities of the Russian garrison in Kaliningrad, and the proposed network of prepositioned munitions and weapons stockpiles will require a major review of how NATO intends to execute contingency plans from Tallinn to Varna."
https://mwi.usma.edu/baltic-trainspotting-railways-natos-logistics-problem-northeastern-europe/