If the solder splashes are on the tops of the rails I would take a light file. wipe the flux residue off with some alcohol on a Q-tip. Depending on where the paint is and what color etc. I wouldn't be too worried as if you plan to weather the track your going to paint it anyway.then clean the tops of the rails with a bright boy.after it drys.
I wouldn't use a Dremel to clean track with anything other than one of their polishing pad/wheel.
Marlon
See pictures of the Clinton-Golden Valley RR
There's an industrial product called "Cratex", it's a rubber bonded abrasive in a wide range of grits and shapes. They've got cylinders, cones, disks, and so on. They can be shaped with a silicon carbide stick like you'd use to dress a bench grinders wheel with or you can let them shape themselves to the work as they wear. The same sources you'll find if you Google "Cratex" will have mandrels you can use in the Dremel or motorized shaft tool.
Lou
My first instinct (which means this is probably a thoroughly horrible idea) would be to take a soldering iron that was good and clean (so as not to add to the problem) and try smoothing out any bumps of solder with a fairly quick pass (to avoid unsoldering any joints). Some cotton soaked in cold water -- a heat sink in other words - on the joint you want to save could be a good idea. That heat should also address any left over flux. Failing that I'd try an old (meaning one you won't be using any more for fine work) chisel type xacto blade to slowly scrape at the solder hopefully without causing the kinds of abrasion that can cause poor contact over time. If you are daring, filing the solder down until you get close to the rail and then letting the chisel blade take over might be faster. A Dremel tool sounds like an invitation to unintended consequences to me, but perhaps using a wire wheel with brass wire (so that the wire wheel is softer metal than your nickel silver rail).
Dave Nelson
Use a fine file followed by some 400 grit wet dry paper if there are any file marks (generally file marks mean you are filing too hard or using too coarse a file.
Dave H. Painted side goes up. My website : wnbranch.com
A good flat file and some emory cloth sandpaper will work fine. Just remeber that wherever you "rough" up the original finish of your track, it will accumulate dirt in that location first.
I can't believe I didn't thin of this before as it's sitting on a shelf staring me right in the face. Go to Radio Shack and get that little container of solder removing braid for under $5.00 you heat he rail where the big ugly splash is and touch the braid to it and watch it wick right up the braid. when your done snip off the used piece and throw it away. You can ever try a solder sucker also available form Radio Shack and apply heat with a "CLEAN" iron and suck up the excess solder. A good reason why your tip should always be clean and to also practice your soldering before you solder rails etc.
Steel Man I am currently laying my track and noticed after I glued it down that I have some very small but noticeable solder splashes along with some flux residue and paint.
Small blobs of solder are not usually adhered to the metal very well. Many times you can remove them simply by sort of chipping them off. If you use something made of soft metal, such as a brass door latch plate, you can use it as a chisel and tap it with a hammer to chip the solder off.
Flux will come off with rubbing alcohol or a mild solvent.
Fresh paint can be removed with paint thinner, or if it's well cured you may need a stronger solvent.