"To railroad" means to rig a situation such that events can only play out in a particular manner, or to a particular end. When applied to a trial, it means to manipulate the judicial system such that a defendant is virtually guaranteed a conviction. The metaphor derives from the nature of a railroad track, which does not offer a train the ability to choose its path of travel.
"The lead prosecutor railroaded Eddy into a first-degree murder conviction: she relied heavily on the testimony of unqualified experts."
"If you are hosting a murder-mystery party, avoid railroading your guests into quickly solving the case. Give them freedom to pursue tangents in the storyline they find interesting."