Hello new here ....
and gonna start by asking few questions
what would i need to make Led circuit with reversing
polarity? ( the power is Ac ) used for model trains
( most the i use for normal lighting is square bridge / cap and
led strip - includes resistors on it already .... )
if i wanted to make red and white leds to switch based on polarity how would i do that?

 

Welcome, but we need a more definite description. Please ramble on.

 

Typically if you are using an LED with A/C, you connect a rectifier diode across the LED (anti-parallel) so that reverse polarity current goes around the LED. But there is more to it. You need to current limit the foward current to a safe level.

 

If it runs on AC, how will you determine when it is going forward or backward? Most of the more advanced engines have a circuit board in them that 'knows' what's going on - that's where the action is.

If you always have AC at the track, it won't be changing with motor direction - or am I missing something? *Shrug*

You CAN (and probably should) use DC for your lighting, however, and change polarity for lights - I did this for my father in law's train with good results.