。
The train was about to arrive on schedule.
Literal
Train [topic-は] on-time [adverbial-に] arrive [was-about-to-ところだった].
~ところだった attached to the non-past form of a verb means 'was about to X' or 'was on the verge of X-ing' — an immediate-future reading set against past tense. Subtly different from ~そうだった ('looked as if it would'). Here the implication is that the train was on track to arrive on time, without committing to whether it actually did.