Even though it might have been the wheel set that fell off the train and jammed in the tracks in the fatal accident, I'm still betting that it fell off because the bolts were not tightened and safety-wired like the rest. And they can't just come out and say it, so they took a highly visible act to "solve" the problem.
When you change a tire you need to torque the lug nuts when you are done. If you change a tire on your car and drive off with the lug nuts only finger-tight against the wheel, I can guarantee you will have far more serious trouble in a very short time.
I can't predict exactly how or where - Might happen 5 miles down the road, might make it 500. Might just go all wobbly and give you a chance to pull over (and call a tow truck because both the wheel and studs are now toast), might shear the wheel studs and drop your brake rotor on the highway at speed as the loose wheel bounces into oncoming traffic and kills someone (through their windshield sideways)... But it's going to try and come off.
--<< Bruce >>--