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,,,,,
Cheers,
Frank
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,,,,,