Good news!  I removed the shell to check things out.  Couldn't see anything wrong, but simply re-positioned the lead wires and now the light is working well.  If it stops working again I at least know where the problem lies, but hopefully we're all set now.

Thanks again for the help!
Steve

Many thanks for the debugging suggestions.  My work schedule has prevented me from giving it a try, but hope to get some time in the next day or two.  Will let you know how it turns out.  Thanks again!

I am new to model railroads and just bought a Thunder Chief DCC diesel locomotive set for my son.  Nearly everything seems to be working, however the Function 10 (turning the lights on/off) I have only gotten to work once. 

Some of the other functions were a bit intermittent, but in the last day all of them have been working reliably with the exception of turning the lights on/off.  They are always off.
Function 8 (mute) has always worked.  The sound functions (1, 2, and 3) initially were a little unreliable, but now seem to work all of the time.  The function 10 only worked during one session (during that session it worked reliably). 

I've checked all of the track connections and everything looks good, re-plugged everything.  Gone through several unplug/power cycles etc.  I haven't re-programmed anything (everything is at factory default).

Wondering what could be the issue? Bachmann support has suggested that I see if the light has gotten twisted around (I'll have to look at that tonight). Any comments/help much appreciated.