My process is to build as I go, and pause when I need to really develop something. I have a notepad section in my Storyist documents dedicated to worldbuilding: what I’ve already built, things I want to develop, things I still need to figure out. Having it in a searchable document is, I think, preferable to handwriting it since it’ll be easier for you to find later on.
That said, let’s chat about worldbuilding. Some things to keep in mind:
You’re building a world for a story. You are not building a story to fit a world. The world should work for the story and serve as a storytelling tool. Even in realistic works, things like consistent seasons and weather, locations, and cultural norms need to carry through into the story. And small details matter, as well. Don’t get so caught up in the planes/trains/automobiles question that you forget to develop culture, food, fashion, all that.
Maintain the mystery. Don’t give it all away in one shot. Characters and narrators don’t have to explain everything—nor should they. Let things unfold rather than bashing the reader in the face with everything you have.
Rule of Cool does not make the world go round. Yes, it’s cool. But does it belong? Part of worldbuilding is deciding what fits and what doesn’t, and this should weigh into your pros and cons. Yes, the robots are cool, but if you’re writing high fantasy, you’d better have a damn good reason for them. If not, say goodbye for the sake of a logical world.
Everything builds on everything else. Worldbuilding is not Pokémon evolution. Things don’t change independent of everything else, they influence each other and build upon each other. Consider how the cars or trains might influence other things in your world: trade, travel, communication, day-to-day life, infrastructure, population density…
(This is where I take your example and run with it.)
A good world is built for a population, not around a protagonist.
Something you might like to consider when deciding between two worldbuilding things is to figure out how each thing will be used/abused/gotten around, and what its most useful application is within your story. Find the loopholes. Figure out what people who love it will say about it, and then what people who hate it will say. There are always two (or more) sides to nearly everything in the world.
So, let’s check out the idea of travel. High speed trains and cars serve very different purposes, and we have the real-world experience to prove it.
Consider places in the world that have adopted the bullet train: Paris, France’s Métro system operates beneath a fairly densely populated metropolitan area. Citizens use the train to get to and from general areas, and walk the rest of the way. Cars are not as commonplace in Paris than they are in other parts of the world because they’re not an absolute necessity; some people walk to the train station, ride, walk to work, and repeat the process back home, and they do just fine. Going even further, France’s TGV (Train à Grande Vitesse [really fast train]) has shortened international travel to most of Europe to a matter of hours, allowing Parisians and many others to travel to and from other countries with ease. However, high speed trains can be incredibly cramped, ill-maintained dependent on funding and infrastructure, and can inconvenience you when they break down, especially if they are your sole method of transport. Building a rapid transit system will take a long time from start to finish, potentially meet some resistance in the form of budget and location (and possibly protest groups), and will be incredibly expensive. Blocking off one of the rails can range from “annoying” to “devastating,” depending on how it’s done (one person on the tracks vs. an entire protest movement launching boulders at the train cars).
Now consider cars. Cars fall short of bullet trains in that they travel slower, have lots of competition on the roads (between pedestrians, bikers, other cars, horses, maybe even some pegasi, I don’t know your story), and that they cost more in that a person either has to own or rent a car before they can drive it regularly. (And then comes the cost of fuel and maintenance cost.) They can be easily broken into and/or stolen, can be very dangerous in the wrong hands (drunk drivers, speeders, someone on the wrong side of the road), and can harm the environment. On the other hand, cars excel over bullet trains in that they can go off-road, do not have set schedules or destinations, can be privately owned, and are more useful for household projects and moving. In a car-centric culture, more people are likely to know how to drive cars, and there will almost always be one nearby. Cars are also more personal than a train will be: on a train, there are innumerable strangers that might be peeking at whatever you’re doing, where cars are nice and small. In times when fuel prices get high, people may opt out of cars in favor of things like high-speed rail (despite the fact that bullet trains use fuel, too). And then there is the ever-present question: where do you park? And what if you do if there’s truly nowhere to park?
Consider also what might fall in between. In this example, that might be something like a park-and-ride structure or a bus system. In addition to this, something beautiful about fictional worldbuilding is that you can create alternatives to whatever you think you’re stuck between, as well. This might mean bringing in pegasi and riding dragons, solving the energy crisis and making completely green cars, or something as simple as expanding a train system to mean smaller trains, bigger train maps, and more stops. There are lots of possibilities even if you’re not writing fantasy.
So: Cars are useful for transport between places of varying lengths, are good for adventuring, and tend to be privately owned and maintained. High speed trains are useful for transport between distant places, are public, and tend to be funded by governments. Neither of them are inherently better or worse, just useful in different ways. This is going to be true of a lot of things, and that is fine. Very rarely will an entire population of people agree that Thing A is unequivocally better than Thing B.
Pros and cons is probably what it will ultimately come down to, but the best way to do it is to put the things into the context of your story. Step outside your protagonist’s field of vision and check out how other people would live with one, and then the other.
Always remember, worldbuilding is a process, not a step. You may never be 100% done building your world. And that’s all right—the real world’s not done either.