The mystery of the Blue Train

MONSIEUR LE MARQUIS, an elegant gentleman who merits his mask but not his white hairsDEMETRIUS PAPOPOLOUS, a venerable dealer in unique antiquesZIA PAPOPOLOUS, whose resemblance to her father, Demetrius, is more than physicalRUFUS VAN ALDIN, a hard man, with plenty of hard cashMAJOR RICHARD KNIGHTON, Van Aldin's secret