Arduino powered from motor shield and USB issue I am using an Arduino (Elegoo) Mega with Motor shield to drive a digital train set. The Motor shield is powered from a 12v 1.1 amp dc power supply and I have the Arduino plugged into my laptop via USB. I have an app which sends commands via serial to the mega which are then turned into commands for the train set and sent down the line.

When using this all of a sudden the main chip on my Arduino blew up and my Laptop seemed to blow a re-settable fuse shutting off instantly (It was dead for about 10-20 minutes then worked again).

Would this be because something caused power to get back through the Arduino to my laptop? 

The railway will short from time to time if a locomotive derails for example.

I know there is a trace on the Motor Shield which prevents the shield powering the Arduino. I have now cut this although it says you only need to cut it if using over 12v.
Looking at pictures of the Elegoo Mega 2560 they use a "hash fill" for the pour on the tab of the regulator, not a solid fill, and zero vias to another dissipation layer on the reverse. While this may look more funky it provides only a fraction of the heat dissipation of a solid double-layer fill, meaning the regulator is more likely to overheat. Also because this is a Chinese clone the component quality will be lower (cheaper) meaning the regulator probably doesn't have proper thermal overload protection.

So it gets too hot too easily, and pop your Arduino is dead as the regulator fails "short" and 12V gets pumped directly into the rest of your Arduino.

Byebye Arduino, and byebye computer (fortunately temporarily in the case of your computer).