they are relatively easy, but very expensive ... a more inexpensive solution is individual leds and appropriate resistors ... that way it's easy [ just vary the resistor] to vary the leds light output
Wehonest_US is shipping from California. Slightly higher pricing but faster delivery. Prewire LED w/resistor for approx 66 cents each in bundles of 15 for $9.99. FREE shipping but now Ebay charges your state tax.
They also have a bunch of other RR related stuff.