There is no such thing as a transitive sentence.
Verbs are transitive or intransitive, not sentences.
"to train" is a transitive verb. As such, it can be used both in active and in passive sentences.
The sample sentence illustrates a transitive verb used in a passive sentence (i.e., a sentence in the passive voice).
In the active counterpart "Someone trained these agents to kill",
agents
is the direct object. But sentences in the passive voice do not
have direct objects because the direct object from the active sentence
has been used as the subject of the passive sentence.
CJ