Through the Lock — Carol Otis Hurst

Etta and her brother and sister, wards of the state of Massachusetts in 1840, have been parceled off to separate foster homes. Determined to gather her family back together and create a secure home for herself and her siblings, Etta has run away from the last in a long string of foster homes, resolved to find a place where they can live as a family. In her search, she encounters Walter, a young runaway who is hiding out from his alcoholic father in a cabin on the New Haven and Northampton Canal.
Before she can get her own family together, Etta gets caught up in Walter's problems, which include the transportation of a body up a mountain in the dead of night, convincing the canal company to hire them as a security team, and catching the vandals who are determined to sabotage the canal.