So how do I fill it?
How big is this gap? If it's just a small gap (up to 1/8th of an inch) pieces cut from an credit card and secured in place with Super Glue works wonders. If larger than 1/8th of an inch cut some small pieces of rail and set them into the gap. Secure with Super Glue.
Dr. Frankendiesel aka Scott Running BearSpace Mouse for president!15 year veteran fire fighterCollector of Apple //e'sRunning Bear EnterprisesHistory Channel Club life member.beatus homo qui invenit sapientiam
The first recourse should be to widen the gaps on either side of the one that you want to close up. If it can be done (one or both rails can be slid), the quickest way to improve the situation is to move the rails together by about 20-30% of the existing gap, hopefully not to compromise too much the gaps on either side in so doing. I have often found that my gaps on either side are nice and tight, something like 1/32" or less, so you have quite a bit of play room if you want to reduce a gap nearing 4/32" between them.
As Jeff has suggested, many of us take sprue from a styrene kit and file and shape it to a usable insert. Use epoxy or some such cement to get it to stay put securely, and then shape it by watching a truck flow across the join. A finger rubbing lightly back and forth on the flange path and on the rail head will help, too.
For gaps that somehow end up 1/4" or larger, you have a choice between simply re-cutting a new piece of track, ties and all, and installing it, or cutting a tiny bit of rail and either soldering it into position or using epoxy.
I don't worry about gaps that are less than 1/8". Even though I'd rather they weren't there, the fact is they don't affect rail traffic except when they are on the tightest curves your equipment can reliably handle...or less...and when you haven't properly "dressed" the ends of all rails cut either by the factory or you. That means taking a small metal file and beveling the top surface and also the flange surface. Additionally, gaps in the order of 3/32" - 1/8" will open and close somewhat with changes in humidity in the wood supporting the whole works, including frame members. When you can see that the largest gaps stabilize for several months at the wider extreme, then you can address them.
The gap is at a soldered joint at a track/switch junction. So redoing the section of track would be not my first choice. (of course if it's my only choice.....)
So I have a narrow slot at the bottom because of the joiner. It looks about "credit card width wide" (n gague) so maybe I'll try that first.
I tend toward cutting a piece of brass and soldering it in the gap, then use needle files to sculpt it to shape. It's similar to the styrene/credit card solution, but I think it may be more permanent.
Phil, I'm not a rocket scientist; they are my students.
I'd solder in a piece of rail. To do so, clamp a short length of rail in a vise, then file away most of the base that extends, on both sides, beyond the web. Use a razor saw or cut-off disk to slice off a suitable length of the modified rail, then insert it into the gap. Because there's probably some solder in the bottom of the rail joiner, the head of the short filler piece will sit higher than the surrounding rail. However, when you touch the soldering iron to the top of the filler, the heat will transfer through it, melting the solder below and allowing the filler to drop into place. You may have to keep the heat applied while you adjust the position of the railhead laterally, but when the joint cools, you'll never know that the gap was there.
Wayne