Do you have a track nail that you could measure? I have purchased self threading screws in 304SS as small as 1mm (.04") on Ebay. Just looked and there are many suppliers. At a glance there were 100pc lots for a couple of dollars.
I second that... You can find steel 1mm self tapping screws up to 5mm long on ebay. I've used them to install N scale flextrack and turnouts on a small vacuformed plastic layout base (I rebuilt the track for someone) and they worked great.
The 1mm diameter is a little larger than the holes in N scale ties, but you can drill them out to clear the self tapping screws. J.I. Morris used to manufacture 0.8mm self tapping screws, but they're super expensive (20 cents per screw or more).
Jeff