Cut for spoilers for anyone who wants to guess the plot of this post for themselves…
The woman (Mrs Landers) is dead, discovered by another train passenger. The mark on her neck indicates to the police that it may not have been natural causes.
The police find a blackmail note in her bag (demanding that she hand over an expensive necklace) and a photograph depicting the necklace. It’s not meant to be a wedding, just a fancy ball, but yeah it does kinda look like one 🙂
Yep, the only person to notice anything on the tram is a little kid, who noticed another lady with Mrs Landers.
Not pictured: the body is taken away for testing and we find out she died from being poisoned.
The police go to Mrs Landers’ house and have tea with her housekeeper, who says that she’s been home all day. She shows them the necklace in question (in Mrs Landers’ room). The police note that she has a coat like the one described by the kid on the tram.
The police go to the pub where the blackmailer had requested Mrs Landers meet him. He’s the dude from the photograph. He’s like ‘yeah, i’m bad but not that bad.’
Not pictured: The barman confirms that the dude has been there all day and therefore couldn’t have killed her.
The police then return to Mrs Landers’ house, find a syringe in the bin outside, and catch the housekeeper leaving the house with the expensive necklace in her bag.