there are actually 3 versions used by LGB, 5 volts, 18 volts and 24 volts. The 5 volt units are driven from a regulator that varies from loco to loco. Some of them actually use 5 volts and the smoke is a little weak. Others have bumped the regulator to 6.4 volts and the 5 volt units smoke much better, about the same as an 18 volt unit that is really running at 18 volts.
The units are actually made for LGB by Seuthe who offers even more types.
What kind of loco is it? If it is regulated unit, then it should run pretty well at lower voltages. If it is one of the non-regulated units, you will have to get the loco moving right along to get any reasonable smoke.