Russians are desperately short of ball-bearings, and they have the choice to keep the trains moving, or repair tanks to continue the war in Ukraine. Since the trains are what moves the tanks to the battles, it's a catch 22

“In 2020 Russia imported over $419 million worth of ball bearings, around 55 percent of which originated in Europe and North America"

Since the invasion of Ukraine, and related sanctions, there is a shortage of a lot of things, one is ball bearings. The other is optics, which used to come from France, for the new production of 72 and 90 Russian tanks.

It’s for that reason that Russia has struggled to make good the 2,000 or more tanks it has lost in 14 months of hard fighting in Ukraine. Russian forces need at least 150 new or restored tanks a month just to maintain their front-line strength.

Yes, there were small stockpiles of ball-bearings but Russian rail-operators needed those bearings, as their 13,000 locomotives moved more and more replacement men and equipment to the Ukraine front.

Which is why the Russians are reactivating 1960s tanks that have been in storage since the 1980s.

https://www.forbes.com/sites/davidaxe/2023/04/19/whats-perfectly-round-made-of-metal-and-keeping-russia-from-replacing-the-2000-tanks-its-lost-in-ukraine/?sh=70ca0ccf23f2