Now, why would you do that instead of relying on your bombers, boomers and nuke fields to end the world? Because it's the Cold War and being paranoid pays the big bucks.
You see, the USSR might just be good enough with their first strike counterforce [^1] (the title of my upcoming arcade shooter, donut steel, TBD) to leave you with missiles that can barely wipe out 50% of Eurasian population - or as MAD/strategic deterrence people would less flippantly say, USSR might feel like it can pull that off, which increases the odds of pulling something.
That's why you get the Peacekeeper Rail Garrison, the US edition of nuke on train. Naturally, the Soviets invented the idea, probably because of some genetic memories of armored trains in the Russian Civil War.
Once the shit is about to hit the fan, you ring the bell, the doom trains leave their protected depots (I think that's what the slide illustrates), and go off into the national rail network. There, they can fire whenever given the order. Since it's a lot harder to nuke the entire rail network or individual moving trains than easily-spotted-from-space silo fields, the enemy can't be sure they'll get them all in one go.
