Any sane PTC system (or whatever their name, considering there are a number of them in Europe, Japan and other places) is constructed so that failure of software or electronic components mean that trains stop. If the system isn't robust, as often happens when it's new, this leads to operational headaches, but safety-wise this is not an issue.

PTC type systems have been around for decades. Germany, Sweden, Japan and other countries where these systems are employed have excellent safety records. Accidents due to failure of these systems are rare indeed. Zero accident rate is a noble goal but in the real world we have to prioritize what controls we will use, or costs will prevent trains from rolling at all.