Reports said the forward train "lost power" after a lightning strike, and the following train hit it from behind. Obviously, the power was not lost in the sense that the overhead wire stopped carrying electricity, because the following train was still running. Probably the lightning strike fried the internal electrics of the locmotive and prevented it from using the power to move.

However, presumably they have PTC, why did that not stop the following train? Or was the PTC system also fried by the lightning strike? Or was the following train so close that PTC was ineffective?

I saw a TV report with a newscaster doing a standup. Some distance behind him, you could see the viaduct, running across the screen from side to side, with the wrecked train cars lying on the ground next to it. As he spoke, a train went across the viaduct, not very fast, but definitely moving. It appears that the officials, after having cleared the viaduct, inspected the track, found it good to go, and allowed traffic to resume.