LED operations in loco

[RhBBob]

My limited knowledge of LED is about to be revealed.....

 

One of my Swiss locos, Bernina 602, has blown one of the forward LED in the number 2 end.  In the standard lighting configuration of 3+1 this is one of the two that are on only in that forward direction.  The remaining light, on a different circuit is always on whether as 3rd headlight in forward mode or tail light in reverse.

 

I replaced it with a slightly (physically) larger one and it seems to have blown the other forward LED and shorted the lighting circuit. I don’t do things by halves!

 

I removed both the new LED, which still works out of the loco, and the 'blown' one and the remainder of the lighting then returned to normal.  Mercifully the ESU v4 chip is still fully functional.

 

So, please, what governs two LED on the same circuit? Do they have to be equal to each other? And can they be different from the remaining, factory-fitted LED on a different circuit ?