LED question

It's not uncommon to see an LED go dim, but they more often just fail completely.

Illumination threshold voltages often vary, so when LEDs are wired in parallel, differences in brightness may happen.  This is why I prefer to wire LEDs in series, which ensures identical LED currents and brightness, absent deterioration.  in parallel, if each LED has its own current-limiting resistor, the brightness differences are minor.

Back in the 70's there was a tri-color LED with only 2 leads that was red with one polarity, green with the other, and yellow with AC applied.  I experimented with them for HO block signals.  I looked for any currently for sale, but had no luck.  Do they still make these?  I see multi-color LEDs, but they have 3 or more leads now.

Identical LEDs in parallel should be pretty close in brightness.  Both MTH and Lionel wire tons of LED's in parallel in various products.  Obviously, wiring with balancing resistors is a superior scheme since slight differences in current curves will not affect your lighting, but with truly identical LEDs, it shouldn't be necessary.

FWIW, I wire all my dual headlights in parallel for any upgrade, it's never been a problem.

For marker/class light LEDs PS-1 were parallel wired.  MTH moved to series markers for PS-2 and 3.  For the LED lights of PS-3 they are parallel for the doubles.  G