。
The train was about to depart.
Literal
Train [topic-は] depart [about-to-ようとしていた].
~(よ)うとする is the volitional + とする pattern, meaning 'try to do X' or 'be about to do X.' In the past continuous (~としていた), it indicates something was on the verge of happening. The related pattern ~ところだった carries a similar 'was about to' meaning — the two overlap, but ~ようとしていた emphasizes the nascent motion toward the action.