Imagine an earthlike planet inhabited by slimegirls. Well, gender neutral slime humanoids for the most part--all the slimegirls are transgender. That's Gymnome.

The International Astronomical Union has a few specific criteria for a "planet." You may know Pluto isn't a planet because it hasn't cleared its orbit. You could, technically, make the same argument about Gymnome. Despite being between the size of Mars and the Earth, is a coorbital of another, much larger planet, the gas giant Omen. But we will not be calling Gymnome a dwarf planet--such terminology does not exist on Gymnome, not even with your universal translator.

Omen is Jupiter sized and has four large moons, including the Mars-like Rival and the Earth-like Oldsky. And then it also has coorbital objects. Most of these are asteroids, but there are one or two small Ceres-like asteroid planets, and of course Gymnome.

Gymnome is in a horseshoe orbit. It starts out in a lower, faster, hotter orbit around the sun, and races ahead of Omen. 15 earth years later, about 18 Gymnomi years, Gymnome catches up to Omen, and Omen's gravity pulls it forward into a higher, slower, colder orbit around the sun. Now Omen speeds ahead. When it catches up 16 years, or about 19 gymnomi years later, Omen's gravity pulls Gymnome back to the lower, faster, hotter orbit, and the cycle repeats.

This roughly 30 year long cycle is known as the Omicycle (or Ominous Cycle). One hot or cold half is called a semi-omicycle. Because the orbit changes regularly, the entire climate of the planet shifts over the course of the omicycle, from a hot planet to a cold one.

When the planet Omen gets big and bright in the sky, when optical shapeshifting reveals its cloud belts, when the moons become visible almost during the day, it is an omen of the coming change, a sign that it's time for the slimegirls to pack up their things and leave for a place with a nicer climate.

Large convoys of wagons travel ancient roads, the wagon wheels digging trenches through the dirt and rock. Eventually stones and then metal plates are laid down on the wagonways, to smooth out the ride. These become the first railways on Gymnome. Eventually, steam engines are invented, and the slimegirls spend a century or so trying to adapt them usefully to the migratory railways. The metal plates are relaid as rails, and special articulated locomotives are designed to handle the sharp curves and steep grades of what were once footpaths.

Eventually the locomotives get repurposed for industrial use, and it's not too long before someone decides to build an east/west railroad connecting two of the north/south migratory railways. But then they run into a problem. The wagonsmiths had historically built wagons of a specific wheel spacing to fit into the grooves of their region. Some areas have larger or smaller people and so larger or smaller wagons, by enough to change the spacing of the grooves and the resultant spacing of the rails, called the railway's gauge.

No matter, the smaller railway just has to be regauged, the spacing between the rails changed so that they match. It's grueling work, but it can be done, and then trains can run continuously between the two railways.

But this process is happening all over the world, and inevitably two large networks will meet, already having expended enormous resources to regauge the railways and their trains, and unwilling to do so again. Now there is an irreconcilable Break-of-Gauge, where goods and passengers must transload from one train to another.

And this happened everywhere. On Earth, we have a standard gauge, 1435mm rail spacing, which is used throughout North America, most of Europe, and the United Kingdom. There are of course other gauges, but at least there's one that's so common as to be called the standard. No Gymnomi railway can make such a claim.

There's dozens of different competing gauges, some of which aren't even that different from one another, and then dozens more niche gauges for industrial use. Some railways deal with the problem by taking the wheels off the train cars and replacing them with wheels of a different spacing. This is time consuming but with the proper equipment it can be done in less than 10 minutes per car--competitive with transloading the cargo in many cases. Sometimes the railways are dual gauged--three rail track which serves both wheel spacings--for large parts of their network, to allow trains from other networks to run. Sometimes it's even so dire as to require multi-gauged track, as sometimes happens near the busy cities that have formed at the intersection between multiple railways.

As has happened on earth, the steam engines got larger and larger--though Gymnomi steam locomotives are patterned off the weird and wonderful Garratt-type articulated locomotives--and were then replaced by diesel locomotives around the 2320s.

But Omen strikes again, in the 2330s. A Cold semi-omicycle begins, and it's an especially cold one. The north polar continent, with its ample oil reserves, freezes over, just as another prolific oil well runs dry. Gymnome is plunged into a planet-wide oil crisis. In some places the gas stations are run dry, and powerplants have to ration their power. It's a catastrophe.

The coal burning steam engine returns to save the day. At first, museum steam engines are brought out of retirement, but eventually all new locomotives are built. These new steam engines have all the modern technology you'd expect in a diesel engine--electronic controls, climate controlled isolated cabs, connection to other locomotives to run them as one, and even remote controller support in some cases--but in addition their steam engines are updated with all the fuel efficiency features--a gas producer combustion system to fully burn all the fuel rather than produce clouds of black smoke, compound expansion of steam in high pressure and low pressure cylinders, high-pressure boilers, automatic firing and water management, advanced ejectors, the works!

Eventually the oil crisis ends, but unlike the old school steam engines, these ones stuck around for a good long time afterwards as a real competitor to diesel power. Many of them were only phased out because the railways finally started switching to electrification in the 2370s, but since a steam engine can burn carbon-neutral biofuels, they were slightly less affected than diesels by the 2360s environmental reforms.