Learn Croatian: Je li vlak jučer kasnio? - Was the train late yesterday?

It uses the phrase Je li at the very beginning to signal a yes/no question. Je acts as the helper verb for the past tense, and the main action word kasnio comes later in the sentence.

Is 'late' an adjective in Croatian like it is in English?

No, and this is a common trap! In Croatian, being late is an action verb: kasniti. Because it is a verb, we put it into the past tense form here (kasnio) instead of using a separate word for 'late'.

Why does kasnio end in -o?

Because the past tense action word has to match the gender of the subject. The subject is vlak (train), which is a masculine noun. If we were talking about a feminine noun, it would end in -la.

Could I put the words in a different order?

Yes, word order is flexible in Croatian. While Je li must stay at the start, you could say Je li jučer vlak kasnio? or Je li kasnio vlak jučer? without changing the meaning. Putting the subject and time in the middle is just a very natural way to phrase it.