A lot of the violence is filmed practically, with most of it being realistic dummies and props. This is to save time later on, as well as being as realistic and close to reality as possible. Many scenes where people get hit by trains or cars, or shot in the head, use anatomically correct models that react in the same way as real humans.
Everything in the film was made incredibly realistic. From the science experiments to the medical procedures, there was always a professional on set making sure everything was as realistic as possible.