The block system doesn't give control of the train's speed. They still allow an accident like the one in Philadelphia -- overspeed around a tight curve.

The modern systems generally prevent this. See [1] for an exception -- the engineers forgot to install the limiting markers at the very end of the high speed line, allowing the train to overspeed into the normal speed line.

[1] https://en.wikipedia.org/wiki/Santiago_de_Compostela_derailm...