Try some baking soda, it will neutralize acid in the flux. You can mix baking soda with a little water to make a paste. Use a tooth brush to clean the joints with the paste. Rinse it off with water. I'd try that on a couple of those solder joints and see how it works, making sure it doesn't harm your track or anything else involved. Pretty sure it will be fine, but I have never tried this on any type of track, better to be safe than sorry.
Never mind, I didn't see you post earlier about the track all being laid already. It doesn't look too bad for being there for 7 years. I think I would just let it go and see what happens. It's also possible you could end up moving, changing the layout, or other layout rework before it all rots away anyway.