The following description was given by Eduard Kornfeld, a Holocaust survivor, who is still alive and well. You can read his story in full at
https://www.vice.com/en_us/article/a37kbk/portraits-of-europes-remaining-holocaust-survivors
Begin quote from the website above:
Eduard Kornfeld was born in 1929 near Bratislava, Slovakia. He was taken to Auschwitz and several other concentration camps. On April 29, 1945, he was freed by American troops from the Dachau camp in Germany, weighing only 60 pounds at the time. His mother Rosa, his father Simon and his siblings Hilda, Josef, Alexander, and Rachel were all murdered in the camps. Kornfeld arrived in Switzerland in 1949 and was nursed in Davos, Switzerland for four years as he recovered from severe tuberculosis. Later, he trained to become a gemstone setter in Zurich. He has two sons, a daughter, and seven grandchildren.
End quote
The following quote is the words of Eduard Kornfeld:
“We were deported in a cattle car, the journey took three days. When the train suddenly stopped, I heard someone shouting outside in German, ‘Get out!’ I looked out of the carriage and saw Nazi officers beating people they thought were moving too slowly. A mother wasn’t moving quick enough because she was trying to take care of her child, so the officers took her infant and threw him in the same truck they put the old and sick in. Those people were sent to be gassed immediately.”
End quote

My photo of a Nazi gas chamber at Auschwitz
How does Eduard Kornfeld know that the people on the truck were “gassed immediately”? Was there a sign that said “This way to the gas chamber”?
Did the Nazi officer say: “Listen up, you stupid Jews. We are taking these people to the gas chamber, but we are allowing you to live, so that there will be Jews who can tell the story in future years.”
I lived in Germany for 20 months after World War II, and I met many German men. I was amazed at how nice they were. The German men were very polite and charming. I assume that they were also polite and charming as they led the Jews to the gas chamber.