I have seen these cameras...they come in pens, watches, buttons, etc. Most of them are very cheap in quality and price. The good quality ones come with a high price ($500++). I would recommend getting a micro 2.4 Ghz or higher security camera (wireless) from a company called Swann security.
Here is their camera, the BlackHawk - it is all enclosed with a rechargeable battery. It is small enough to be pulled or pushed by a locomotive and you have live video streaming to any TV with 3 AV jacks (yellow, red, and white).
http://www.swannsecurity...ducts/view/?product=358
Search around -- you can find them cheaper.
Good luck,
Paul