There's your problem...

What you described isn't uncommon with QSI decoders.  They have a reputation for randomly crapping out if you leave them for any length of time.  Not everybody experiences this, but sounds like you're one of the lucky ones who did.

flash memory is not forever memory

I'm not an expert on flash memory or anything, but I believe it depends on a static charge to control the gates in its transistors and thereby store information.  Depending on the quality of the materials, these gates can maintain their charge for years, but there are things which can happen to discharge them prematurely.  Leaving a flash device idle for too long will eventually cause its memory to be lost.  Perhaps the design of some decoders make them less reliable than others.

Did something happen?

You said one of the decoders fried just sitting there.  Is it possible that the house was hit by lightning, or that some sort of power surge?  I can imagine a scenario in which some sort of surge could cause a spike on the rails, reprogramming or frying decoders.

Was the layout plugged in all that time?  Were the locos on the rails?

As a non electronic anecdote,

As a non electronic anecdote, you need to thin the herd by 12 locos or get your butt in the train room and play more often, or a bit of both. Seriously though, wait, that is actually serious. I have had issues as well and no where near 6 months, more like 6 days. I have never fried one just sitting there, but I have had to reset everything for some reason, and come to think of it, it seems like they always were the QSI.  Decoder Pro helps in this case. Sorry I am not really helping, but lean on me brother.

Steve       

Did something happen? - not that I know of.

Ken,

I'm not aware of any major event that could have fried the decoder and there were also 10 other locos on the layout (on the rails) that weren't affected.  When the railroad isn't in use I turn off the power to the controller (Digitrax).  Everything is protected by surge protectors.  It's a real mystery....

Rick

Short circuits are a problem

I've read that if you have short circuits on a track it can affect locomotives on the same circuit, whether they are running or not.  If you have had locos derailing in the same block or something similar causing the booster to have to kick in and out, it can sometimes cause decoders to lose their programming.  It should not cause any damage, though.  I imagine you have tried to do a decoder reset on the one which doesn't respond.

Have you visually inspected the decoder?

It seems strange that a decoder just sitting on the track would short out or burn up, especially if no other decoders on other locomotives sitting on the same track died.  It may just be that it either had a solder joint fail or a bit of corrosion on an electrical connection causing an open circuit.

Short circuits

Jurgen,

You are correct!  I have some Athearn Gensis F7 locos with sound and sometimes when they run through a turnout they "lose their mind" and have to be reset!

Rick

Decoder inspection

Russ,

Yes, I did inspect the decoder and the "magic smoke" was released apparently in an area on the board that is burned.  I suspect that the decoder was defective and thus the failure.

Rick

A Reason For Everything

Burned means heat. Heat means current. For some reason too much current passed through the decoder at the smoke leakage point.

Non-Smoking Decoders

Somebody has to invent a casing for decoders to make sure the smoke stays inside.  Every time it gets out it's a bad thing.

Kevin