。
Are trains run on oil?
Literal
Train [topic-は] oil [instrumental-で] cause-to-run [question-のですか].
走らせる is the causative of 走る ('to run') — 'cause/make to run,' the standard way to say 'drive' or 'operate' something that runs. で marks the fuel/means. The explanatory ~のです(か) frames the question as seeking background understanding, not just a yes/no fact.