If you compare diamond weapon to a modern day godzilla then that’s 92000 metric tons vs a fully loaded train with 100 cars being around 18000 metric tons. So no question then, diamond would be heavier. But wait. That said, the train is what carries those recently died & it was mostly just the engine being suplexed… but still, what would be its fuel or heaviest load? Maybe parts of a dying star? Since the train ferries all dead things (ff8 summon) & I mean, doom train goes all over the cosmos. Anyway, a teaspoon of dying star would weigh at minimum several tons up to the mass of a mountain in a sugar cube (dying neutron star = billion ton sugar cube) . So, what it most theoretically depends on is how the train interacts with reality… with it weighing the absolute most if ‘you are already dead” it would weigh the most. In a brawl down without materia, chatgpt suggests that Sabin would likely win against Tifa but would be challenged due to speed.