Utilitarian consists the idea of maximizing the total benefit and happiness and reducing the negatives. In the case of Trolley car, preventing the death of five seems better than prevent the death of one, which is consistent with the idea of Utilitarian— maximizing the happiness. However, it is actually hard to determine which option is better because we’re not sure which choice will bring more utility and benefit with the uncertainty.
In the case of the expert surgeon, it is similar to the situation of Trolley car — five lives versus one life. However, the situation here is five dying people versus one healthy people. Without the permission of the healthy person, the behavior of using his organs to save the other five people’s lives can be definitely viewed as immoral. From the perspective of Utilitarian, making one healthy person live could bring more utility than saving the other five. Because the other five people are recognized as dying, and it’s an uncertainty if sacrificing one healthy can save the five. To maximize the utility and happiness, killing that healthy patient would definitely be wrong.