The explosion derailed the last three cars on the Nevsky Express, a high-speed train traveling to St. Petersburg from Moscow. The bomb consisted of up to 15 pounds of explosives and detonated under the second-to-last car.
A Neo-Nazi group claimed responsibility, but an attack by ultra-nationalists has been downplayed by many Russian security experts because of their lack of resources and expertise. They say that careful planning and execution point to separatist militants from the North Caucasus region.
"It was well-prepared, well-planned, the target was chosen very well," said Carnegie's Petrov. "It means it was done by some organized force and by good professionals."
The Nevsky Express is a popular train that ferries officials, executives and tourists between Russia's two biggest cities, Moscow and St. Petersburg. One former deputy governor, a federal official and six foreigners were among the dead.
"What is very important is the fact that [the attack] was directed against political elites, that's why this train was chosen," said Petrov. "It was designed to have a huge impact and attract a lot of attention."
Parallels have been drawn with the 2007 bombing of the Nevsky Express which failed to derail the train or kill anyone, though around 60 were injured. Andrei Soldatov, editor of the security web site Agentura.ru, called the latest attack an "improvement" on the methods used in 2007 and said it's possible that the same group of Islamist rebels was involved.
Two men from Chechnya's neighbor Ingushetia have been charged with the 2007 attack, though the man believed to have orchestrated it, Pavel Kosolapov, is still at large. It is unclear whether authorities suspect him of being behind this bombing.
The war against Chechnya's separatist insurgents is officially over, but violence in the North Caucasus has spiked this year with almost daily reports of clashes between security forces and rebels. In an attack similar to Friday's, train tracks in the republic of Dagestan were blown up Monday, though no one was hurt.
The bombing is Russia's biggest terrorist attack outside of the North Caucasus since two passenger planes were downed in August of 2004. More attacks like last week's are possible, Soldatov argues, because the structure of the militant groups has changed, now consisting of smaller cells of three to five people that are more effective.
Russia's security forces have instead focused on preventing large-scale attacks that could politically destabilize regions of the country, he says, rather than smaller attacks that don't pose a threat.
"That's why there's no need to change the system because there is no threat to political stability," says Soldatov.
Nikolai Petrov agrees that attacks similar to Friday's causing "huge damage, huge losses" could happen again "due to the fact that Russian railroads and Russian authorities didn't learn anything from the previous attacks."
Meanwhile, the attack has shocked the Russian public, with moments of silence held over the weekend for the victims. Many agreed with the experts that that this could lead to a spate of attacks.
"It's tragic," said Alexander Nagornykh, a 53- year-old engineer. "I think this is the beginning of the new series of terrorist acts…I don't travel a lot but I think that the number of people traveling by train will go down."
Social worker Maria Sokolova said she wouldn't stop traveling, but admitted, "I'm afraid." Seemingly undeterred, the Nevsky Express was back up and running just two days after the attack.