。
It was because of the storm that the train stopped.
Literal
Train [subject-が] stopped [nominalizer-のは] storm [of-の] fault [copula-だった].
Cleft construction: [nominalized event]+のは+[explanation]+だ. The 列車が止まった part is nominalized with の and topicalized with は, then the whole sentence becomes 'the stopping of the train was the storm's fault.' せい ('fault, cause, the reason') typically frames a negative consequence. Contrast with おかげ (for positive outcomes).