It's well outside their range, their range is supposed to be 40km, they'd have had to be fired within Russia if it were struck by them. Also, they're not all that powerful, used for targeting vehicles.
No details on the other loitering munitions recently supplied though & what their range may be but they were said to be tactical ergo I'd assume also not particularly powerful.
Ukraine does have some Tochka missiles that might have been able to reach it and possibly the recent supplies of equipment from the US could have included missiles capable of reaching it too (though that would be a bit of a risky escalation if they've basically just been handed something by the US specifically to hit these targets within Russia).
They have had those helicopter raids recently too at Belgorod, might have tried one of those again - remember they were also flying helicopters under Russian noses in and out of Mariupol for days too.
Possibly it was a super ballsy air strike by a fast jet too, though this seems less likely than a helicopter raid.
Lastly, some Ukrainian SF types or collaborators operating within Russia or indeed some Russians acting out of their own interests - there have been some mysterious Oligarch deaths, various arrests or military & security types etc.. recently - possibly some anti-Putin or anti-Ukraine invasion group within the Russian military or security services might actively sabotage things within Russia completely unconnected to any Ukrainian operations there.
Bryansk has had rail derailments too recently AFAIK.
