The Holocaust trains were hired by Adolf Eichmann, and the Reichsbahn demanded one-way fares be paid by the victims, although children below the age of four were allowed free travel to their deaths.
https://en.wikipedia.org/wiki/Resettlement_to_the_East
The SS manual covered such trains, suggesting a carrying capacity per trainset of 2,500 people in 50 cars, each boxcar loaded with 50 prisoners. In reality, however, boxcars were routinely loaded to 200% of capacity or 100 people per car. This resulted in an average of 5,000 people per trainset.