You mentioned not watching much tv & didn't volunteer anything about books earlier. There are an absurd number of tropes that fit with it, like different flavors of what I expected to be one general trope. Just to start with a couple....This...
...doesn't lead to this.
If Jacqueline loves some NPC, that has nothing to do with the PCs trying to build a lightning rail. No PC can make an NPC love Jacqueline back, so that's off the table and it seems extremely unlikely that her desire for an NPC would actually interfere with a PC's plans. If that NPC was integral to the PC's desire to build a railroad, having her fall in love with him would actually be a boon, because she's so politically powerful she could get the NPC whatever funds he needs in an effort to woo him.
Now, if the PC and the NPC were in a romantic relationship, that might be a thing, but so far I've never had that sort of relationship come up. And it would be so obnoxious--"she wants your boyfriend and is gonna do bad things to you if she doesn't get him"--that I wouldn't want to run that plot anyway.

Murder the Hypotenuse - TV Tropes
Love Triangles are tricky business. Trust us, we've already done the math. It can get particularly twisted when some sides of the triangle are completely one-sided. Alice is in love with Bob, but Bob not only fails to realize it but is already in …


Stalker with a Crush - TV Tropes
A character noted for their obsession with another character. They claim that the motivation for their morally outrageous acts is a simple wish to gain attention from the crush-ee. Alternatively, the obsession may be explained as a misguided …

Jaqueline loves Andy... Andy is really into the circus because of that one performer & now the PCs have two problems
PCs are trying to build a damn in the river, Jaqueline loves Chuck... Chuck is a merman.... oooops
Jaqueline loves Dave... Dave is the BBEG.... oooops
Any number of crazy spawned problems can spin off from her being in love with some other NPC & the worst part for the PCs is there is literally no way to predict it since she can just find some new NPC to stalk