Question about shafts, rods, and broken parts...

I was looking at Carlos Llamas' pictures again. Every time I do, I end up thinking about stuff my wife would say I shouldn' think about.
In any case, I was hoping somebody real, real smart would be able to tell me why rods get bent on locomotives. When 463 busted a rod (last year?) it seemed pretty self evident -- too much pressure on a weak part caused it to break.
But, I'm looking at Carlos' picture of 484's left side main crank pin without the eccentric and I wondered why this isn't all put together with keys and cotter pins that would break instead of breaking bushings, shafts, or rods. Does somebody know why some of these things are fixed joints, instead of with keys or pins as pressure relief devices?
Thanks,
Tom