The Travel Twins and the Lost Secret of the Vikings — Charles A. Salter

Twelve year old twins Josh and Hannah travel from the USA to visit their Uncle Olaf in Norway. Olaf runs the Museum of Norse Antiquities in Oslo, and the museum's greatest treasure-the Viking chieftain Leif Erikson's fabled Sunstone-is stolen. Hot on the trail of the suspected thief, the twins ride on a dizzyingly high train through the Scandes Mountains to Norway's largest fjord, a deep waterway which connects directly to the North Sea. While spying on the suspect's suspicious behavior, the twins are suddenly kidnapped on the train! They elude their captors by breaking free, sneaking across the top of the train as it chugs through a tunnel, and tricking the kidnappers. Then they learn the thief's true motivation: to use the Sunstone to interpret the coded Undredal Runes so as to re-create the Viking super-warriors known as Berserkers. Can the twins defeat the plot, return the stolen treasure, and save the world from the secret of Viking Berserkers which has been concealed for a thousand years?