A trio of mysteries from the Detective Book Club. Reviews from Goodreads.
Compartment K (Helen Reilly, 1955). “It had been planned, everybody thought, as a family weekend. The Canadian Rockies, a lodge--it was something to look forward to. Until an uninvited guest became part of the group. A man of mystery. A stranger. But there was no mystery about this: as the miles faded, a killer stalked the night and the stranger became a corpse. And he was, it turned out, not such a stranger after all.”

Night Drop (Frederick C. Davis, 1961). “A slimy type with the unlikely name of J. J. Junius, Jr....and a dead type who had fallen to the sidewalk from a much greater height than was good for him...and a meek, clerk-like type, obviously up to no good. What did these three have to do with each other? Schyler Cole and Luke Speare, private detectives, had to find out - and quick.”

Double Image (Roy Vickers, 1955). “Five stories by Roy Vickers, starting with Double Image - A man invents a spurious double in order to pin a heinous crime on his non-existent twin, supposedly separated at birth, and continues the charade until even his own wife is fooled by it.”