For some reason whenever there's a terrorist attack in Russia my first thought is always Chechens, or others from the Caucasus. Which doesn't necessarily make sense - you may very well be right about ISIS. It would be a change of targets, but hardly an illogical one.
Either way, it's awful, my heart goes out to the victims and their families.
And given its actions in Syria, it's not a bad guess. Russia has labeled it terrorism so now we wait to find out who was behind it. Given how ISIS likes transportation targets, I would not be surprised. If it were a Chechen in 2017 I would be surprised.
You are right that it is awful. I feel so sad for all the victims - Paris, Brussels, Nice, Berlin, London and now probably St. Petersburg. I know there are lots more in the Middle East too but I tend to Eurocentric.