A Russian agent working with the Atesh partisan movement carried out the attack. The agent refused to take payment.
From: Kyiv Post by Kateryna Zakharchenko

Ukrainian partisans reported that one of their agents blew up a railroad track near the Russian city of Yekaterinburg – about 1,600 kilometers (995 miles) from Ukraine – that was being used to transport North Korean ammunition to be used for Russia’s ongoing Ukraine invasion, the Atesh underground movement said Friday, July 5.
An Atesh spokesperson told Kyiv Post that a Russian agent working for the partisan movement carried out the sabotage.
“He’s refused to be rewarded because he works for ideological reasons. His goal is to stop the criminal war waged by his country against Ukraine,” the spokesperson said Friday afternoon.
The partisans said that journalists are barred from the scene – however, information about the attack has been leaked on social media platforms.
“It is interesting that only after receiving a command from the regional FSB office, the repair team was allowed to arrive at the site and begin restoration work,” the guerrillas said, adding: “We would like to remind you once again that railroad explosions will happen even more often as long as ammunition is being transported.”