The Kabane themselves. Being almost literally made of iron, they can tear through iron bulkheads with their bare hands. At least in other works, you can block a zombie behind a door. And that isn't even getting into how these things can apparently come up with basic ambush tactics. No wonder most of the population collectively shits themselves at any Kabane attack.
The first train crash that leads to everything is played horrifyingly straight. Seeing a train literally brimming with kabane along it's sides, the station hands try to raise the bridge only to realize it's too late. The entire iron train derails, crashing into the station and flattening a quarter of the houses, houses filled with people just heading to bed. Then the Kabane start emerging...
Several Stations have fallen to the Kabane, but due to the massive work the whole train system is, they could not redirect the rails. Hence trains from each station must pass by abandoned stations filled with Kabane with almost each trip. And not all of them make it.
The first Wazatori. It not only knows how to fight properly with swords, but it also shows a sadistic side not seen in Kabanes yet. Against a victim that can't fight back, the Wazatori took its time to pin her down with one of its sword, then proceed to shred her with the other sword while she screamed in terror.
The Black Smoke:
A massive creature made of what appears to be thousands of kabane, this thing alone destroyed yet another station.
It later turns out these are bio-weapons made from Kabaneri females injected with a stimulant.
In episode 8, it's shown that the Kabane were lured to the city. How? With a horse with three corpses tied to it. Given that they were strung behind the horse, it's implied that the three were alive and forced to run behind the horse to draw the kabane to the city.
