Switzerland's trade was blockaded by both the Allies and by the Axis. Each side openly exerted pressure on Switzerland not to trade with the other. Economic cooperation and extension of credit to the Third Reich varied according to the perceived likelihood of invasion, and the availability of other trading partners. Concessions reached their zenith after a crucial rail link through Vichy France was severed in 1942, leaving Switzerland completely surrounded by the Axis. Switzerland relied on trade for half of its food and essentially all of its fuel, however the Swiss controlled vital trans-alpine rail tunnels between Germany and Italy and possessed considerable electrical generating capacity that was relatively safe from air attack. Switzerland's most important exports during the war were precision machine tools, watches, jewel bearings (used in bomb sights), electricity, and dairy products. Until 1936, the Swiss franc was the only remaining major freely convertible currency in the world, and both the Allies and the Germans sold large amounts of gold to the Swiss National Bank. Between 1940 and 1945, the German Reichsbank sold 1.3 billion francs (approximately 18 billion Francs adjusted for inflation to 2019) worth of gold to Swiss Banks in exchange for Swiss francs and other foreign currency, which were used to buy strategically important raw materials like tungsten and oil from neutral countries. Hundreds of millions of francs worth of this gold was monetary gold plundered from the central banks of occupied countries. A total of 581,000 francs' worth of "Melmer" gold taken from Holocaust victims in eastern Europe was sold to Swiss banks.