The Lionel lamp drivers are strange and will not drive
LEDs directly. Try connecting a 12v filament lamp across
the output then connect the LED and resistor in
parallel with the filament lamp.
They will drive an LED, but you have to provide the resistive load I mentioned previously. I have many installations with LED lighting using the Lionel lamp drivers.
Note that the ERR stuff apparently has the resistor on the board as they don't require the load resistor.