The panelling inside the train was wood, the exterior frame was all metal. Proven by the fact when Franky uses his bazooka on the train, the explosives weren't able to even dent the surface.
Also, the Wiki specifically mentions that TBone is able to cut
metal trains and the same thing is written for Zoro that he was able to cut
metal train.
Also, 1st CoA was necessary to cut steel and no hakiless swordsmen could do it, and Brook wasn't capable of it. Now that I prooved he did, it must be his DF...
Lmao, Brook's DF doesn't increase his DC. His power is to summon 'The chill of underworld' producing ice whenever he cuts someone. But 1st of all he should be able to cut that someone for his power to work. Also, I don't see how little bit of frost helps him cut steel..?? And whenever Brook uses his DF his sword starts steaming a bit, but if you check the panel where Brook cuts the Dragon, you'll see that he didn't use his powers, it was a simple slash.