Giving Ukraine modern tanks remains a key question facing Kyiv’s allies, but perhaps the most exposed link in supply chains for producing such weapons runs on train tracks through the foe they’re trying to defeat.

Russia is at the center of a rail cargo route supplying Western arms manufacturers with a steady supply of metals needed to make the microchips, electronics and ammunition used in modern weaponry. Most of the so-called rare earth elements are mined in China. Russian Railways JSC and other carriers are hauling a rising volume of critical metals needed for Europe’s defense industry.

The volume of Chinese rare earth metals shipped on trains across Russia surged to 36,074 tons in the first nine months last year, more than double the amount transported in all of 2021, according to European Union data seen by Bloomberg News. The value of that trade rose by more than fourth-fifths, to €377 million ($408 million) through September.