One whole car of the train traveling on Regina (from Hancock to Paradiso) contained a regiment of federal marshals. One of the marshals attempted to stop Malcolm Reynolds and two of his crew from stealing a large shipment of Pescaline D, but was knocked out. The local sheriff, Bourne, requested the assistance of these marshals; his plea was rejected by an Alliance colonel countless miles away in deep space who had ultimate jurisdiction. 
Months later, Agent McGinnis was an important federal marshal and had a small squad of subordinate marshals at his disposal at Saint Lucy's hospital on Ariel. They arrested Jayne Cobb and the siblings Simon and River Tam. One of them was killed by Jayne, while the rest, including McGinnis, were wiped out by the Hands of Blue using a highly-lethal sonic device.