Complex sentence: Kalmenhof, a private mental institution in Idstein gained notoriety through its participation in the Nazi euthanasia programme, "Action T4", serving as a transit camp for the killing centre at Hadamar.
Split into simpler sentences: Kalmenhof, a private mental institution in Idstein gained notoriety through its participation in the Nazi euthanasia programme, "Action T4". Kalmenhof served as a transit camp for the killing centre at Hadamar.