spikes
Micro mark small spikes are my current favorate, but they are a little long, and will stick out of the bottom of the bridge tie which looks messy unless it is trimmed off.
Micro engineering small spikes might be a good bet 225-30106. I don't have an un opened pack, and the ones I have are stuck on a magenet with no positive ID. I know for a fact that the micro spikes are too small, they fold up on the cedar or poplar I like to use for ties. they would go in to balsa fine, but I'm not sure they would hold worth spit.
I dug through my massive pile of pictures, and came up with this one, this is the narrow gauge bridge I am building on a portable layout. that is an experimental layout for me, as it is mostly foam. the whole thread is in the narrow gauge section, my home layout is documented in the logging minning and industrial section, as is our Clarksville Tennessee model railrload club (Bill and Tom's amazing adventure.