Could I ask you to clarify precisely what you mean? I know the passenger service world, not the freight world, so I'll stick to that. A great many designs have changed, for a great many reasons. Sometimes it has to do with safety. For example, the flared out, upside-down smokestack archetypal of our view of an Old West steam engine is really only necessary for wood burning trains (More common than you'd expect. Freight isn't my thing, but I can tell you logging railroads heavily used wood burning locomotives even when most railroads had coal, because why pay to ship coal out into the logging camps when you've got plenty of timber you can use? Coal may be better fuel, but the wood is basically free to loggers compared to the cost of transporting coal), coal burners having a straight pipe. The cone is a spark catcher to prevent fires, which is really only an issue for burning wood. Passenger cars become heavier as time goes on partially as an issue of comfort (less rattling and rocking around, and you can start making luxurious Pullman interiors), and partially as an issue of safety (lightweight wooden cars have basically no durability in a crash, and casualties could be very high). Of course, with more passengers as incomes rise and people can buy tickets, and the population overall increases and people spread out further, well, you need bigger locomotives, especially with these heavier Pullman cars, and early 20th Century steam locomotives are bigger, bulkier, and more utilitarian.

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Then coming into the 20s and 30s we're learning more about aerodynamics and how we can streamline a train to get it to go faster, and you start seeing those really curvy shapes, with hidden smokestacks, and those fairings on the sides of the engine. You also start seeing diesels crop up, and America really gets into diesels with streamlined trains. Very smooth and sleek, meant to go fast, the same basic models can do both passenger and freight. And from here, things really diverge, to where American trains just have a specific look to them, that you don't see in Europe, and I do not know European trains, so I'm useless to say why German trains look like German trains look. America got the EMD E and F units (F3 and F7 being the more well known), which is basically the image of a diesel train in America to this day. I can say in America, as our passenger railroads collapsed and Amtrak nationalized passenger service, those streamlined diesels were old and falling apart, and Amtrak needed a new locomotive, and the main manufacturer was only really doing freight, so Amtrak got basically a retooled freight design. Especially because the manufacturer thought Amtrak would fail, and wanted to sell Amtrak something that could be repurposed for freight if Amtrak did indeed fail. So Amtrak got that truck design with the elongated cab that's common to this day in the US. The EMD SDP40F.

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Honestly, I can't tell you what's up with that kinda semi-truckish shape, as compared to a flat cab. Freight is, again, not my thing. I just know why Amtrak ended up with basically freight locomotives. And they didn't work well for a bunch of unimportant reasons to this story, so EMD made a new design, the F40PF, which still looks like a semi-truck, and despite still being basically a freight design, it's beloved, it served well, I like them, Amtrak sold them off as they aged, and many regional railroads, such as the Caltrain I ride frequently, still have them today (though we are getting ready to retire them). Now the trend is back towards streamliners, because it's clear passenger rail isn't failing and we can afford passenger rail locomotives, and, well, streamlining is just better. It's aerodynamic.

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That said, this isn't really a great answer, because I need more to go off of. I'm just sort of shooting off some basic, surface level examples of changes that have happened over time. Do you want to know more about those Pullman cars, and how those developed into lighter weight cars for streamliners despite more weight originally being desirable? About how a lot of American rail shifted over to two level passenger cars as opposed to one level? Why streamliners went away for so long? Why freight has influenced American passenger rail design for a while? How Amtrak has searched for new locomotive designs? In order to answer the original prompt, I need to know what about the traditional 1800s trains you've seen change, so I can look into why that may have been.