Trains
All aboaaaaaaaaaaaaaaaaaaaard!
Trains were a feature of mid-Season Two, intended to both make travel easier and Places more mechanically interesting to visit. They were exactly what they sounded like: big ol' slabs of metal and steam, plowing their way through the Jungle and flattening everything in their path in the name of public transportation. You'd need a ticket to ride one, and as one would expect they'd normally only stop at major landmarks like Outposts.
But contestants who (literally) played their cards right would be given a special First Class Rail Pass, allowing them to get off at any map tile they desired. We're not sure how the driver managed to make those things go cross-country, but by god did he ever succeed.
Unlike some of Improbable Island's past features,1) we know exactly how Trains used to work. A group of train enthusiasts known as the Cardtell documented the entire system in a handy PDF, which you can grab here. Just look for the “Improbable Trains Compendium” button on the sidebar.
You might think this article is a shameless plug for merchandise, but the truth is that this editor simply hasn't gotten around to copy pasting the entire document yet. But hey, if you want to buy some replica train tickets while you're at it, that sure is also on the page.