@Daron Only if you limit your view to the western front after entrenchment. The early war in the west and the eastern front had successful non-suicidal cavalry charges (after entrenchment infantry charges were largely suicidal yet you don't claim infantry were not a thing). And again, that we are talking about cavalry in 1914 highlights how absurd it is to claim cavalry disappeared because of muskets.

@vsz cavalry charges were still used and were certainly not ended with the development of muskets. It was even effective in the First World War. Wherever did you get the idea that they were suicidal and not done?

@Daron Your point was that pikes disappeared because cavalry disappeared, which is solidly wrong. Had you said pikes disappeared directly because muskets got better, I would not have pointed out how your statements do not resemble actual history.

I suggest you read some military history before making such claims about pikes and cavalry. Pikes mostly disappeared because having more musket men with bayonets were superior to a mix of muskets and pikes, despite the bayonet being inferior in melee. Cavalry were quite a vital part of warfare well into the 20th century as even lancers (guys on horseback armed with a pointy stick) were still effective in the first world war (it was barbed wire and trench warfare stopped them on the western front while the eastern front still had plenty of cavalry action).

do you have any evidence for that claim? The industrial revolution in Britain started ~1760, which is nearly a century before steam locomotives became dominant in transportation. Most products are not perishable, so taking longer to get there is not a significant issue (cloth doesn't spoil easily). Furthermore there is no requirement for fast locomotion for concepts such as refrigeration to be developed, and canning significantly predated steam travel. Textiles were one of the biggest drivers of industrialization as it was such a large market - perishable food on the other hand wasn't.

@Goodies The claim that wood is somehow effectively fossil fuel is still nonsense - it's a renewable resource which is readily farmed. The real-world existence of successful commercial charcoal-based steel production, despite the availability of cheap coal, demonstrates overwhelmingly that charcoal steel production would be easily viable without coal. People have asserted that rapid fossil fuel based travel is necessary, but any demonstrable evidence is wholly lacking as there is no such requirement (as evidenced by the industrial revolution starting a century earlier than steam engines).

@AlexP Yes they had a relative abundance of coal making it a more convenient source of energy than charcoal for the most part. So what? Other areas like the US or Russia had an abundance of wood which made charcoal relatively cheap and easily available. The early industrial revolution was primarily powered by water - steam power came later after industrialization was well underway - and there is no requirement that a civilization be short of wood for them to industrialize.

Where did this idea that industrialization means fast travel (instead of industrial production) come from? Rapid transport is only necessary for moving highly-perishable goods long distances - that has nothing to do with industrialization. Non-perishable goods (which is almost everything) and alternative for transporting perishables like changing them into less-perishable forms (refrigeration, canning, turning milk into cheese, etc.) do not require rapid transportation, and none of that has anything to do with industrialization. Mass-produced clothing or machine parts aren't going to spoil.

I don't see why charcoal wouldn't be used as a fuel source, at the very least iron smelting would need the carbon to make steel anyway. Iron-burning engines are too far-fetched for an early industrial civilization (unlikely for any level really), and completely unnecessary anyway. Even with fossil fuels horses and sailing ships still dominated until the late 19th century, which is quite late in terms of industrialization.

Almost everything here is entirely wrong. Wood is not a fossil fuel. Mobility is not a requirement for industrialization. Even industrial scale steel production can run on charcoal (last one I know of didn't shut down until 1945). An industrial revolution is about industrialization - changing the mode of production from bespoke artisan products into efficient factory-processed goods available in abundance - it has nothing to do with some absurd specific definition of making vehicles which burn fossil fuels (how did you think that was the definition of industrialization?).

I see little justification for defining an industrial revolution as being exclusively about fast land transportation. More rapid mobility certainly allows greater trade of highly-perishable goods, but that is it. I fail to see how "fresh milk" is a requirement for any industrial activity (and keep in mind that the industrial revolution had already been underway for well over a century before the development of railroads). Steam power itself was a later development spurred on by the industrial revolution outstripping the ability to use water power.

Helium will not mix into the other gasses but will rapidly rise to the edge of space (or beyond), so the effective Helium composition of the atmosphere will always be negligible until the very outer edge regardless of total planetary composition. That stratification renders the percentage on the planet as a whole beyond practical considerations (unless one wants to picture the airships floating on the transition between layers).

By definition airships rely upon lighter-than-air gases for lift - that is what makes them airships. That you have to assume the requirement of a complete change of atmospheric composition along with any changes in gravity reveals your flawed argument - given the same gases, higher gravity makes Helium-filled airships more efficient. There is no requirement for different gravities to have different atmospheric compositions as they are independent factors. Furthermore your point that Helium is not a lifting gas in a Helium environment is both tautological and entirely irrelevant.

High gravity makes flight easier for airships, not harder. The higher air pressure means lifting gasses are even more effective.

Not sure why you feel the need to specify your atmospheric content, but the helium is irrelevant for any practical consideration as it doesn't just mix together with heavier elements. Helium is sourced by tapping trapped air pockets underground (byproduct of natural gas wells), because it rises to the edge of space (or escaping entirely) instead of thoroughly mixing with gases such as Nitrogen and Oxygen. For Helium to be such high concentrations in the lower atmosphere it would need to be constantly bubbling up everywhere.

@user3067860 You don't need to prop anything up, just don't inhibit as different modalities are useful for different purposes (i.e. most passengers are best sent by plane or bus, while bulk cargo is best sent by ship, with train catching unusual circumstances like middle-volume cargo or high passenger volume into dense spots). As far as "where a train could run", it would be appropriate to consider wherever a bridge/tunnel could possibly be built, which with modern technology is most places (they just don't bother when a ferry service is better than building a bridge or tunnel).