May not have, a decoder, You didn't say if it was DC or DCC,,, in any case, take the shell off and you will find out quick enough,,,,,
Cheers,
Frank
If you can get the shell,off without destroying it,,, my guess it is a 1.5 volt bulb, hard wired to the decoder,,havn't seen a screw, in bulb, since I stopped believing, in Santa Claus,, LOL Miniatronics,has bulbs that will work in that,,,,,