In your Lionel stuff with LEDs, AC is being rectified to DC before it hits the led lamp.
Like Stan says knowing more he can produce a more "ideal supply".
But more than likely, if its a well built unit, it just needs a bridge rectifier, or a few diodes to run off the transformer.
Capping the max voltage might be important too. A small voltage regulator might be needed too, and that's where design, while still basic, needs "more" added, based on the light unit.
Ask the supplier for a spec sheet, at least min.-max. voltages, etc . If they don't have one, they are likely "middle men" & not close with the mfg, so no telling what you'll have. Might be just a lamp, or it might be on a switching board.
$5-$20 is my guess. Once done, it can still run on dc power well too.
