Yes, they did pack them tight on the trains, and many died there, too. I'm not sure "half", but it was a lot.

Not just packed in tight. No room to sit, let alone curl up on the floor to sleep. No food. Almost no water. One large bucket or garbage can for the entire car of people to use as a toilet, and never emptied at the occasional stop. Children, young people, elderly and old people. Healthy and sick. Some of the people who collapsed got trampled. The windows were opened only the tiniest crack. The stench of urine, feces, and death became unbearable. And then when the train of cattle cars arrived at their destination and those who survived the trip left the cars, they were lined up for decision-making time. Those who ran the camp when down the line directing some people (children, the ill or infirm, the elderly) to the left, and those reasonably healthy and strong to the right. Those sent to the right were taken to have their hair shaved off, hosed down, given a striped uniform, and sent to a crowded barrack. Those sent to the left were killed.