I assume it partly depends on how much lateral velocity the phone had. If it landed at a good angle with respect to the train and "slid" away most of it's energy, then perhaps nothing unusual would be noticed.

Anyways is it the obvious thing to lock a phone merely because of a large jolt?

Boeing doesn’t make trains. This phone fell out of the sky.

Yes, and planes stay in the sky because they travel and hundreds of miles per hour of relative ground speed. So, you would expect things that fall out of them to have a lateral velocity component in addition to their vertical one.

A jet airplane is low-drag and has thrust from its engines; a phone has neither. The phone’s downward velocity is constantly accelerated of gravity, so it will stay at terminal velocity downward. Its forward velocity begins rapidly decreasing immediately upon leaving the plane, due to air resistance. After just a few seconds, the phone will be traveling mostly-downward.

Same/similar shock as dropping the phone. Should it lock on drop? Weird feature to add.

A locked phone is exactly what you want in an emergency /s

Emergency functionality works irrespective of lock status.

"Situation is an Emergency for you" !== "Best action is to use your phone's Emergency functionality".

Of course, however I am imagining a car accident where a phone was displaying maps/music/etc suddenly locking itself. The screen being lit up could save valuable seconds in finding it when it’s tossed around the car

But judging by the downvotes there is something I am missing with this