It is to be the fourth and probably final major memorial to the Nazis' victims built in or near Berlin's central Tiergarten park, following sites dedicated over the last decade to Jews, gays and Roma slaughtered in the Holocaust.
"The murder of tens of thousands of patients and residents of care homes was the first systematic mass crime of the National Socialist regime," said Uwe Neumaerker, director of the memorial foundation.
"It is considered a forerunner of the extermination of European Jews."
The site next to the city's world-renowned Philharmonie concert hall will commemorate the fates of people like Traub, who was admitted to a psychiatric hospital near the Dutch border in 1931.
Nine years later, with Hitler at the height of his power, he was selected for transfer nearly 300 kilometres (190 miles) away to a Nazi "intermediate facility" in the western state of Hesse.
In 1941, he was taken to a clinic nearby in the town of Hadamar which had been transformed into a factory of death. There, immediately after his arrival, Traub was sent to a gas chamber and murdered with carbon monoxide.
His parents received word from the clinic that their son "died suddenly and unexpectedly of flu with subsequent meningitis".
Because he suffered from a "serious, incurable mental illness", the letter continued, the family should see his death as "a relief".
- An elegant villa -
In an elegant villa at Tiergartenstrasse 4, more than 60 Nazi bureaucrats and like-minded doctors worked in secret under the "T4" programme to organise the mass murder of sanatorium and psychiatric hospital patients deemed unworthy to live.
Between January 1940 and August 1941 doctors systematically gassed more than 70,000 people -- the physically and mentally handicapped, those with learning disabilities, and people branded social "misfits" -- at six sites across the German empire.
Protests by members of the public and leaders of the Catholic Church ended the T4 programme but the killing went on.
From August 1941 until the war's end in 1945, tens of thousands more died through forced starvation, neglect or fatal doses of painkillers such as morphine administered by purported caregivers.
The German parliament voted in November 2011 to erect a memorial to the victims of the Nazis' cynically labelled "euthanasia" programme where the villa once stood.
The Berlin government selected a design featuring a 24-metre-long (79-foot-long) transparent blue-tinted glass wall on a dark base.
Adjacent panels will detail the history of the T4 programme and its victims. The ensemble will be accessible to the disabled.
- Lacking a 'strong lobby' -
The T4 programme was only one part of an orchestrated campaign to eliminate the "unfit" and the memorial is intended to commemorate the hundreds of thousands of victims, many of whom died with the tacit consent of their families, Neumaerker said.
Few of the killers were brought to justice after the war, despite high-profile trials like those of doctors at Nuremberg 1946-47, and many of the implicated medical professionals simply continued with their careers.
Meanwhile both West Germany and the communist East did little to recognise or compensate survivors.
Smaller plaques and markers have been installed at relevant sites across Germany in recent years but the T4 site is the first national memorial to honour these victims.
Berlin's daily Der Tagesspiegel noted that unlike other groups, the "euthanasia" victims lacked a "strong lobby".
"Many were forgotten for decades and still are, even by their own families," it said.