The mod Railway Motor Car (0.4.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event railway-motor-car::railway-motor-car-key (ID 247)
Given entity doesn't exist anymore.
stack traceback:
[C]: in function 'newindex'
__railway-motor-car/scripts/use_equipment.lua:111: in function 'mount'
railway-motor-car/scripts/use_equipment.lua:169: in function <railway-motor-car/scripts/use_equipment.lua:159>

This happens on 100% of attempts to put down the motor car and enter it. I had been using the mod very well for 2 days, in the same world. Didn't make any changes to my mod list. Used the motorcar to drive out to one of my mines. Went to enter it again to drive back home, and crashed. Crashes again each and every time I try to enter it. That's disappointing because I really like this mod. Hope the stack dump provides you some insight.

Note that I'm running on linux64.
Factorio 1.1.87 build 61545 linux64.

Do you have the "Keep schedule"-setting enabled? It may work if you disable it.
I'll look into it, maybe it was broken due to the last update.

I might have, I'm not sure. I've been using it both on and off. Thanks for the tip too.

I could not actually reproduce it, but I fixed an possible exception with 0.4.2