The rust belt is north of Kansas and slightly east. Basically the states ringing the Great Lakes. Wisconsin and Illinois all the way over to western NY.
Manufacturing used to be concentrated in these areas because it was cheaper to ship raw materials (especially heavy and bulky ones like coal or iron ore) in and manufactured goods out over water to the ports and population centers that at the time were heavily concentrated on the east coast.
As an added bonus, the region was conveniently (and thus inexpensively) close to the coal producing regions of Appalachia that produced the fuel that powered the factories.
Once railways augmented the port network, the Great Lakes port towns were some of the first and best connected to the rail system because they were already so important economically, and rail connections reinforced their position.
Especially because even with rail you would save days and dollars getting your goods from Chicago to NY compared to the time it would take to ship them from Topeka to NY - and the same applied to acquiring raw materials.
Rail also allowed towns in the region that weren't directly on the Great Lakes or the Ohio or Mississippi rivers to participate in the boom by building spur lines. Since steel production in particular was concentrated in the region, industries that relied on steel had incentive to concentrate there, near their suppliers.
The de-emphasis of boat and rail for shipping, and their replacement by trucks in the second half of the 20th century wasn't the deciding factor in the collapse of these towns' economies, but it didn't help either because it eroded their existing advantages.
Edited to add (though this is already long): the erosion of job opportunity in the US for the past couple decades has been occurring everywhere other than a few major cities such as New York and SF (and a few outliers like Pittsburgh, which was one of the hardest hit rust belt cities but has recently rebounded as a center for the medical and bioengineering industries).
Cities outside the rust belt (including those in Kansas) have also seen an erosion in opportunity as manufacturing and other mid-skill jobs have left the country or been automated away or further concentrated in the major coastal cities.
The rust belt was just the first region to be hit visibly by the problem, as much as 20 years before the impact became obvious in the rest of the country. It had farther to fall, too, being not just a stable economic region but a powerhouse.