Small LEGO train sets on narrow gauge track?
It might work as is; but if not, you can certainly replace the wheels on 40034 with small train wheels, as seen in about every set featuring narrow track. Chance is, if you've got narrow track, you probably have some of these wheels anyway (unless you got the track from the flying saucer); but in any case these wheels aren't that hard to find. Not sure three sets of wheels in a row would work though, so maybe you'll need to remove the center ones.

For 31015 this might be a bit more complex to replace the big wheels, but maybe regular train wheels might work here. Again, having too many wheels in a row might be a problem as the narrow track curves are very short.