Reply to "Help wiring LED's. Im building/assembling a kasiners Passenger car kit from about 1949 and want to add LED's. But want to power it through the third rail."

Many, many folks here, myself included, use LED strip lights, not individual LEDs, to light passenger cars.   For those running DCS/TMCC/Legacy with 18V in the track, 12V strips are preferred.  For conventional operators, 5V strips are preferred.

Here is an example of 12V, 5 meter strip of 300 LEDs for $2.54 w/ free shipping:

https://www.aliexpress.com/ite...b159-fb7be67657bb-15

Simply cut-off the length needed for a particular car.

Power from the track runs through a regulator like @gunrunnerjohn provides via Hennings Trains (just click on the link in his signature above) or from a LM 2596 AC/DC buck converter available on eBay.

Good luck with your project, especially the brass vista dome fabrication.