Although no dates are specified, the novel likely takes place in the 1950s: Ruthie reads the novel Not as a Stranger, a bestseller from 1954; and Sylvie's husband "fought in the Pacific." Like Ruthie and Lucille, Robinson (born in 1943) was an adolescent in the late 1950s.
Presumably, the three Foster sisters were born in the late 1910s (as Sylvie is in her mid-thirties when the main plot begins) and the train accident occurred around 1930 (as the three sisters were in their early teens at that time). The train accident in the novel bears many similarities to the Custer Creek train wreck of 1938, in which a passenger train derailed from a bridge into a creek in Montana (the state that borders Idaho), killing 47 people. It remains Montana's worst-ever rail disaster.
