僕は電車が遅延したので遅刻した。

。

I was late because the train was delayed.

Literal

I [topic-は] train [subject-が] delayed [reason-ので] was-late.

~ので gives a reason/cause. 遅延する ('to be delayed') is Sino-Japanese and formal — typical of train announcements. 遅刻する ('to be late, tardy'). A standard apology context for being late to work or school.