。
Smoking is prohibited inside the train.
Literal
Train-inside [location-で] smoking [topic-は] is-prohibited.
A classic public-notice register. 列車内で uses the suffix 内 ('inside of') to form a locative noun, then で marks the location of the prohibited action. 喫煙 ('smoking') is the formal Sino-Japanese noun; tobacco-specific signs use this rather than タバコを吸う. 禁じられる is the passive of 禁じる ('to prohibit'), producing the impersonal 'is prohibited' reading standard on signs.