The mod Train Speed Limits (1.0.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event train-speed-limits::on_nth_tick(5)
train-speed-limits/control.lua:66: attempt to index local 'originalRail' (a nil value)
stack traceback:
train-speed-limits/control.lua:66: in function <train-speed-limits/control.lua:49>

I've had the same error, although I found out what was causing it for me.

One of my rail signals with a speed limit applied was targeted for deconstruction. When a train passed into this section, the error would occur.
Screenshot of issue (imgur.com/5K6fUao)

Not sure if OP had the same problem, but thought posting this would help :)

Hi, thank you both for reporting the bug, I've also noticed this same bug a few times myself and I will try to fix it in the next update.

Thanks for the speedy update, love the mod!