Symbolism, Imagery, Allegory
So everyone is pretty stoked about the motorcar Eldric brings to town. Though we might appreciate a nice car now and again, this one little car is a majorly big deal. This is because this car isn't just any old car—it's a symbol of the future. Remember: People are still living in a time without indoor plumbing or electricity in Swampsea, and they can't get to a city without hopping a train for what probably amounts to a very long journey.
So when everyone goes crazy over the presence of this one car, wiping their dirty little kid hands on the leather interior and begging for rides, they are actually showing just how excited people are for change. Symbolically, this is interesting to think about alongside the symbolism of the swamp (which appears elsewhere in this section, so be sure to check it out).
At one point Briony mentions motorcars in a string of things that she says she'd "never have"—"Everything about Eldric screamed of the things I'd never have, of London and theater and turn-on lamps and motorcars—" (2.38)—and when she does, this wicked witch seems to be turning green with envy as she thinks of all the modern conveniences Eldric has access to that she does not.
The car doesn't just represent this for Briony though, but also for the citizens of Swampsea in general. They are all fascinated with this new modern toy, which shows that they are excited with the newness and bustle of the modern world that is being ushered into their lives. Even Rose, who seems pretty disinterested in the things other people find interesting, catches the technology bug when she sees Eldric's motorcar.
Briony notices that on their first drive, Rose holds a "real conversation" (25.28), asking questions about how the car works and where its noises come from (25.38). The answers that Eldric gives confuse Briony, proving yet again that this new modern world is not Briony's place. Where Briony might be mistress and wolfgirl when it comes to spirits and swamp life, she is nothing to this new world. Eldric acts as a bridge in a way then, pulling her closer to this new world by introducing her to things like his shiny red car.