Before the Meiji Restoration (1868) beef was largely avoided in Japan: a 1200-year Buddhist proscription on four-legged meat meant most Japanese had never tasted it. When Japan opened to the West, beef-eating became a deliberate symbol of 文明開化 (bunmei kaika, 'civilisation and enlightenment'). Gyū-nabe — beef stewed with onions and sweet soy in a cast-iron pot — was the first dish to take hold of Tokyo's appetite, and the direct ancestor of today's sukiyaki. Restaurants serving it (牛鍋屋) were the trendy modern venues of 1870s Yokohama and Tokyo.

By the Taishō era (1912–1926) — the setting of Demon Slayer — gyū-nabe had spread from upscale Tokyo establishments to the ekiben (駅弁), the boxed meals sold on railway platforms. The dish lost some of its avant-garde shock value but kept its association with modernity, mobility, and the New Japan of trains, ports and Western suits.

Kyojuro Rengoku eating a gyū-nabe bento on the Mugen Train is therefore doubly modern: a Westernised meal on a Westernised form of transport. Yet his speech style — direct, plain-form, plain-spoken masculine register, the un-ironic 「うまい！」 three times in a row — is pre-Meiji warrior idealism. The contrast is the joke of the scene. Kyojuro is the modern Japan riding on a steam train eating beef, AND the Tokugawa-era retainer who would have refused that beef on principle, in one person. The bento isn't background detail — it's a one-image essay on the period.