For slot car frames, you will need to have an acid flux always. Yes, it is possible to solder brass to brass with rosin flux, but believe me it works better with acid flux, and any time there's piano wire involved, acid flux is a must..
The best acid flux IMO is Stay-Clean:
http://www.harrisproductsgroup.com/en/P ... -Flux.aspx. It is available from many different places, in two or three sizes, so Google for the best price and delivery to your area.
As for solder, most 60/40 solders are fine, but for me, the extra strength from a 2% silver solder is worth it. I use this:
https://www.radioshack.com/products/sil ... 0331827717 or this:
https://www.radioshack.com/products/rad ... 0331574469 The difference is just the diameter of the wire. The smaller one helps with fine work to control how much solder is placed in the joint.
While you are ordering from Radio Shack, get some of this too:
https://www.radioshack.com/products/rad ... 0331827077This is a tip cleaner and tinner for your soldering iron. It works much better than a wet sponge and helps the tip last longer, especially when cleaned off using one of these:
http://www.homedepot.com/p/Hakko-599B-S ... /204264881.
BTW, nice to see that Home Depot are stocking Hakko Soldering iron accessories now - they are super high quality. Just a pity they don't have the right irons for our needs. The best Hakko iron for scratch builders is the FX601, IMO.