Ruth Fielding at Snow Camp by Alice B. Emerson

While on the way to a winter break vacation at Snow Camp, Ruth, Helen, and Tom cross paths with a strangely sullen boy named Fred Hatfield. The chum's chaperone, Mr. Cameron, suspects the boy of being a runaway, and decides to take him back to his family in the town near Snow Camp. When the runaway jumps from their train on the way North, Ruth follows -- and what follows is a harrowing adventure with a panther, a crazy hermit, and a trip through the chilling climate of New York state!