Anyone who would claim that it was knows very little about either hardware or software. When the processor runs, the software executes. When the processor stops running due to cold weather, that's a hardware issue, not a software issue. As you say, designing electronic equipment for its operating environment (with reasonable safety margins for extremes) is critical.
This is both a hardware and a software issue.
There are a series of sensors on the rail line that tell the positions of trains, their velocity, the positions of switches, etc. so that trains can move on the system without colliding. The software considers the input of the sensors and decides whether the line is clear ahead to propel the train. If the line is blocked, the train stops a safe distance from the obstruction.
But what if the software gets no signal from a sensor? The safe thing is to stop the train. But perhaps the practical thing is to let the human operator move the train at a slow speed of perhaps 10 mph if he can see the way is clear ahead and allow the train to get past the dead sensor. Once the train clears the sensor, things can proceed normally again.
That would be fault-tolerant software.