Circumstantial evidence is as valid as direct evidence. Let's say that Todd didn't shoot the kid at the train robbery. The kid rides home and tells someone that he saw three men with a pump after the train pulled away. The cops investigate and discover that the methylmine has been diluted. They discover the tank buried in the ground near the tracks. The kid's testimony that these three guys were near the train with a pump is circumstantial because he didn't see them actually pumping out the methylmine. The fact that the methylmine is diluted is circumstantial (it would be direct evidence except that you can't prove that it wasn't diluted at some other point in the train's journey). The fact that there is a tank large enough to hold the missing methylmine right where the kid said he saw the men is circumstantial. All of your evidence is circumstantial but when you put it together you will easily convict those three men of robbing the methylmine (assuming that the kid IDs them). Circumstantial evidence got a bad reputation from lawyer shows on TV, but it makes up the majority of evidence in many successful convictions. You won't hear "Objection! Circumstantial" in any courtroom except for on TV.