The joke was so unrelated to what actually happened (problems with a train) that it kind of feels as if she was just waiting to make that joke, and decided to use the situation to do so.
It’s a joke which says: I am entitled to judge your behaviour. It also says, don’t call on me for help, I will judge you for it, and you can’t trust me.
A scenario where your baby is taken away is not funny.
It’s like if someone you’re close to suddenly made a lame unrelated joke about smashing your face in with a sledgehammer. You would know they weren’t going to, but the image would still be violent, motivated by their hidden anger, and it would not be funny.
I hate the “it’s just a joke!” brigade. Jokes can be used with malice and as an excuse to express latent aggression.
That said, if it’s a one off, just ignore it for now. But get a taxi next time. She has shattered your trust a bit and she needs to win it back.
Or joke back: “wow with a comment like that you’re going to be an estranged grandparent and die alone in a nursing home!’
Except don’t, obviously, because that’s also a cruel unfunny ‘joke’.