Sorry I am confused as to the need for a 680 ohm resistor across. Not sure of the voltage input of the LCRX. Only if it is AC do you need the diode, If it is DC you do not even need the diode, just a proper resistor. If the diode is in proper polarity it wont hurt anything though.
The 680 ohm resistor is required to give the triac enough "leakage" current to fire, it won't fire without some sort of resistive load. This is common and happens on the R2LC as well.
You can get AC out of the light output in conventional mode, or when the locomotive thinks it's in conventional mode, so you need the diode. I mistakenly omitted the diode in early LED conversions, and after a few months the LED's started dying, apparently from reverse voltage spikes in excess of their reverse voltage ratings.
You need both the resistor and the diode. Also, as GGG says, for some firmware you could reverse polarity on lights, I don't know if that applies to the LCRX as well.