I'd recommend soldering those joints. It doesn't take long if you set up an assembly line. Plus, you don't really want to solder "every" joint. You need to leave a few, say every 6-8 feet of track, with a slight gap in the rails (about as thick as an NMRA guage) and an unsoldered joint to allow for expantion and contraction of the rails due to heat and cold variences. Just solder a jumper wire across that joint so electrical connection is maintained.
Remember, the #1 problem in N scale when it comes to reliable running is maintaining electrical connectivity. Dirt on the rails can be a major issue, but larger then that is that the rails have to have electricity going to them, or all the cleaning in the world isn't going to make the train move across the dead track.
Trust me on this one, there's not much in this hobby thats less fun then loco's that refuse to run well.
Good trackwork can go a long ways towards minimizing those problems.
Besides, mixing up all that JB Weld and applying it, even if it did work, would probably take just as long, if not longer, then just soldering it up right.
Just my thoughts.................