The Golden Rule by Amanda Craig
When Hannah is invited into the First-Class carriage of the London to Penzance train by Jinni, she walks into a spider's web. Now a poor young single mother, Hannah once escaped Cornwall to go to university.
But once she married Jake and had his child, her dreams were crushed into bitter disillusion. Her husband has left her for Eve, rich and childless, and Hannah has been surviving by becoming a cleaner in London. Jinni is equally angry and bitter, and in the course of their journey the two women agree to murder each other's husbands.
After all, they are strangers on a train - who could possibly connect them? But when Hannah goes to Jinni's husband's home the next night, she finds Stan, a huge, hairy, ugly drunk who has his own problems - not least the care of a half-ruined house and garden. He claims Jinni is a very different person to the one who has persuaded Hannah to commit a terrible crime. Who is telling the truth - and who is the real victim?