Just a thought here. An LED is a diode, a one way device. In know for a fact from a Lionel C420 experience, that certain R2LC receivers on the light outputs and maybe even other ouputs can have the Triac only switch on the DC side of the incoming waveform depending on the version (example, C08 VS C07). As such, entirely possible is all you have is an LED and dropping resistor, if you wire it one polarity it works, and the other polarity it doesn't. Might be worth double checking that before assuming the output is dead from the board- or test with an original bulb. Also, might require slight capacitance (Edit- or just resistance via a load resistor) to ensure the TRIAC is triggering. You might be chasing tail when the problem is your LED setup.