Why does this circuit keep killing my 555 timers?
The overheating only happens with 12V, which I'm getting from a hobby
  power supply normally used for a train. My meter reads ~11.5V on the
  supply.


I have a suspicion that the sort of power supply used for a model tranin may be un-smoothed rectified DC. You meter reads 11.5 VDC but the peak voltage may be above the 18 volts specified as the maximum for the NE555DR.

If you used an electrolytic capacitor on the output of the power supply, you could check this fact out. My suspicion is that the DC voltage would rise to something above 18 volts.

The average value of a full wave rectified sinewave is 0.637 x peak value therefore, the peak value for an average of 11.5 volts is 18.05 volts.

It's worth checking.