Learn Dutch: Was de overstap op de stoptrein maar kort! - If only the transfer to the local train were short!

This is a verb-first exclamation. By starting with the verb and adding the modal particle maar, you create a sentence that expresses a strong unfulfilled wish. This structure changes what would normally be a yes/no question into an exclamation meaning 'If only...!'.

Why is Was in the past tense when the translation 'were short' refers to a present or future transfer?

To express a hypothetical situation or a wish that isn't true right now, Dutch uses the simple past tense. Just like English uses 'were' instead of 'is' (the past subjunctive) in 'If only it were short', Dutch uses the past tense was to show that the transfer is, unfortunately, long.

Could I leave out the word maar in this sentence?

No, without maar, the sentence just becomes a literal yes/no question: Was de overstap op de stoptrein kort? (Was the transfer to the local train short?). The particle maar (or sometimes toch) is the essential trigger that turns the question structure into a wish.

Why do we say de overstap op instead of using naar (to)?

In Dutch, you use the preposition op (on/onto) when talking about transferring to another train, bus, or tram. Because the verb is overstappen op (to transfer onto), the noun form takes the exact same preposition: de overstap op de stoptrein.