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"Dangers on a Train" is the twenty second and final episode of Season 24.
Marge stumbles upon a website for married women to arrange affairs (after mistaking it for a website dedicated to cupcakes) and meets a man named Ben (Seth MacFarlane), who falls for Marge after the two bond over a Downton Abbey-esque period drama. Meanwhile, Bart and Homer obtain a small-scale train from the closing Towne Centre at Springfield Glenne shopping center and continue to build up the train.
Homer and Marge are celebrating their wedding anniversary and Homer remembers that their first anniversary was one of the happiest days they ever had, between a visit to an upscale outdoor mall, getting Ned Flanders to watch a hyperactive infant Bart, and a lovemaking session on the mall's mini-train (which was called "Li'l Lisa" and provided the name of our starfish-headed middle child). When Homer heads to the mall to plan another anniversary, he is stunned to see that it is a nearly-deserted pit of failure, but he manages to save the small train and plans to fix it with the help of his bar buddies and Reverend Lovejoy. Meanwhile, Marge tries to order Homer a batch of Dolly Madison cupcakes but accidentally signs onto a site called "Sassy Madison" (a parody of Ashley Madison) that helps married people have affairs, and in the course of deleting many perverted e-mails she ends up talking to a likable man named Ben (Seth MacFarlane). At first, she tries to tell him that she is not interested but ends up sharing a same passion for a series which is Upton Rectory; matters aren't helped because Homer's efforts to hide his train project are alienating Marge. It is only until Homer's plans are finished that he and Marge have gratified their love affair.