I agree with Randy. If you want to operate an LED properly, you need to operate it within the current rating that it is specified for.
In most circuits the voltage is constant, and depending on the voltage of the LED and its maximum current rating, there is one resistor value that maximizes brightness. see LED circuit for determining the resistor value.
But in model railroads, the LED voltage may vary, and a current regulator is appropriate, which limits the current once the voltage exceeds some value. See A Better Constant Brightness Circuit for LEDs which describes a the use of a small 20 ma regualtor. This of course only works if the max current rating of the LED is 20 ma or less