Halliday & Matthiessen (2014: 349):
In a transitive pattern the participants are obligatory Actor and optional Goal; if there is Actor only, the verb is intransitive and active in voice, while if both are present the verb is transitive and may be either active or passive. This is still the basis of the English system; but there is little trace of transitivity left in the verb, and voice is now more a feature of the clause.