I only didn't ask this because Sagan actually explains that part in the book.
If I understood correctly, protons repel each other, and the reason atoms don't usually tear apart is because neutrons exert a force over protons that can counter that repelling and keep the atom's nucleous together. If an isotope does not have enough neutrons to fully counter the proton's repelling forces, it is deemed "unstable" and eventually loses nuclear particles. This is the case of Uranium-238.
Why uranium-238 loses specifically 2 protons and 2 neutrons was not explained, but I assume it's a result of the specific difference between the forces of neutrons holding it together and the forces of protons repelling each other.
Feel free to correct me if I got anything wrong.
Honestly, the perspective of what constitutes a functioning public transit system depends a lot on what you have as a point of reference.
I'm portuguese but I lived in Germany for 5 months during which I used exclusively public transports and bikes. Central Europeans complain a lot about Deutsche Bahn and indeed during this time I saw a few strikes, delays and suppressions. However, transports were still much more reliable and much more frequent than I'm used too so I could never really consider it problematic, although my Central European friends complained a lot.