Hey Desert Center, the light bulb is an integral part of the smoke unit circuit. Here's the Lionel service manual on your caboose:
http://pictures.olsenstoy.com/...onopcar/noc6557a.pdf
The search feature on this forum can sometimes be a little difficult to utilize. I have found Google to be easier and if there are topics on this forum pertaining to the search, they'll show up there. Here's a list of topics on the 6557. I haven't gone through this list, but you may find an answer...