So, this probably is related to the fact that the Soviets used a different gauge of railroad tracks than the rest of Europe. The Soviet tracks were about 5 feet wide, while the standard European gauge was 4'8.5". The Soviets desperately needed coal to power their trains, factories, and for heat. So it would make sense for them to lay Soviet gauge track to the mines instead of transferring loads from one train to another at the border.

The wikipedia page on 5' gauge has some info and if you look around you can find lots of websites talking about the issues with the gauge change and how it hindered the Nazi supply when they invaded. The Wehrmacht has this popular conception that it was highly mechanized but it turns out not to be true. They were heavily reliant on horses and rails. So, when they lost the rail link they began suffering significant supply issues.

https://en.wikipedia.org/wiki/5_ft_and_1520_mm_gauge_railways