Tony, I don't think that anybody actually knows the millicandela rating of their headlight LEDs. I think that you are getting a bit too technical.
LEDs do have a brightness rating (at their nominal operating current), but that doesn't mean that they are operated in models using that current. Most often, LEDs are powered using lower current (as white LEDs are often too bright at their rated current and brightness). The voltage which is applied to the LED headlight circuit (and thus current) also depends on the track voltage (both DC and DCC). Both can vary. That will change the brightness of the LED.
The bottom line is that pretty much any white LED will work for the locomotive headlights. Its operating brightness can be adjusted by changing the resistor value. This is something you can do bu experimentation.
The color temperature is a bit more difficult to match. Not only it depends on the LED phosphor's chemistry, it also can change slightly depending on the current passing through the LED. I would say that for model headlight use, not having exact matching of color temperature is good enough.
BTW, the Atlas Shay uses standard old-fashioned yellow LEDs (not white). That is one of the reasons I replaced the LEDs in mine with white LEDs.
