Ages ago, when I was still fooling with different worlds and styles of play, I conceived of a modernistic campaign based on Top Secret combat rules that was to be set in northern Michigan. Specifically, upon the railroads of northern Michigan. The idea was that gas had grown scarce, but that electric power could still be generated ... and that factions would fight for control over those existing bands of rails that enabled them to carry food from the fields of Minnesota to the Great Lakes. The premise, I admit, was never that deep, and the game never actually got off the ground. Still, I liked the idea of railroad to ground battles taking place, augmented by the need to deliver successfully the stable that would keep tens of thousands of people alive. Naturally, there would also be factions who controlled the switches, to allow trains to move from one track to another ... but like I said, the whole idea never came off.
It is hard, I think, for people to comprehend now the influence and change brought on by the implementation of railroads in the 19th century - nor how fast those rails were laid down. Over a period of 22 years, from 1838 to 1860, railroads expanded from 500 miles of track to more than 10,000 ... at a cost of around $50,000 per mile of track in the U.S. (more in England and Germany). Passenger totals per year increased by more than 25 times, to more than a hundred million. Thousands upon thousands of rolling stock cars were built, coal was suddenly in demand everywhere and the very nature of the country was changed over from what was to what is, as least for us. The line between ourselves and the world of D&D was struck swiftly ... and with it came a remarkable host of social changes. Things we take for granted - regular travel, delivered goods, mass immigration and polyglot society - none of this was conceived of prior to 1830. Travel was something that was done for months at a time, and only by a very few people. The majority of goods that could be bought by most of the population were strictly local. People did not marry far-afield, except in the rarest of cases.
This last is usually overlooked. The sharing of our gene pool, that which we accomplish in a blink of an eye as we marry people from across the country or across the world, has served greatly to improve the health and well-being of the human population. Both pestilence and deformaties thrive in an isolated environment - even if that environment is East Vancouver or the slums of Washington D.C. Diaspora, which railroads enabled, does more than give you or I a greater range of healthy prospects for our choice in partner, it shines a light on localities because we can go there, look for ourselves and bring aid as needed.
Compare the backwaters of Europe prior to the 18th century - those places which did not sit upon the major trade routes, or did not have easy access to the sea or plentiful fertile land. It is no error than Cervantes places his disturbed hero from the land of La Mancha, which sits high upon a poorly thriving plateau where no one with sense would travel. Lands in France, such as Gascony or Auvergne, were infamous for the stupidity - indeed, the mental retardation - of their residents. It would be all well and good to say that this was due to an urban Paris looking down upon their rural counterparts ... but the French habit of headbinding was a RURAL phenomena, and one that quickly disappeared with the spread of railroads into obscure parts of the country.
What the railroads did was to initiate our modern sense of homogeneity in culture - aided of course by automobiles and air travel. The initial effect was that prices in a particular part of the country were quickly leveled, so that the cost of a loaf of bread became virtually the same in Pittsburgh as it was in Des Moines. This has become universal; the coke bottle sitting on my desk as this is written costs the same as the one sitting on the gentle reader's desk as this is read. You may be in Florida or Toronto or San Diego ... it makes no difference. Time and distance have ceased to matter, since said bottles flood into our cities daily, no matter what the origin. The price at origin is increased in order to compensate for the price at destination ... and we accept this as a matter of course.
If you intend to create an economic model for your D&D world, you must begin by throwing all this out. Speed of travel doesn't exist at all. It is not even a comparison of a train travelling at 60 miles an hour compared to a cart that manages 8 miles a day. Railroads, for all their headaches, still suffer fewer delays and difficulties than do wagons on even 19th century roads. Rail placement is far more flexible than canal works - which in any case are unusual in pre-Reformation Europe, though China had more than a few - and coal makes a better, more resilient fuel, less subject to spoilage, than forage. Animals die, secondary roads become mires, foreigner's lose their way and it is much, much easier to plunder wagon goods moving along a highway than rolling stock upon rail. The question is not how fast do the goods arrive, but indeed if the goods can be expected to arrive at all.
In short, it is not what railroads may do for your world, but in fact what they have not done yet. Populations are infinitely more xenophobic, cultures more inward looking and distrustful, scarcity more rampant, intercultural relations often non-existent except for the purpose of war and thus innovation non-existent. Remember too that the lack of travel meant far fewer people being educated at what were far fewer institutions of learning. It was only the rich man's son who had any chance of being educated even by Jesuits - the only widespread institution of learning in Europe - or by Moghul or Manchu/Ming beauracracies. Less instruction means a dumber, more ignorant populace, or one that is probably less willingly corrupted or swayed by high-minded player intrigues or explanations.
Don't forget that most of the time the peasants should look rather uncomprehending at parties most of the time, as said parties do their usual song-and-dance about what they're doing there and what they've come for.
These people are not citizens of the world, after all. You could argue, and rightly, that there is no "world" as we understand it ... just a big unknown that begins on the other side of far ridge.