I haven't soldered to Gargraves (my new layout to be is Ross track, similar), but what I found worked for me was pretty much what GRR said
1)got the tinplate off the area to be soldered (this was lionel old O27). Found that it took more work then I thought, plus the rough contact area seemed to help.
2)I used rosin core solder but I also used paste solder, too (might be overkill, not professional, but I did it).
3)I tinned the wire to be soldered and I also tinned the rail.
4)I then used the iron to hold the wire in place, let it heat up, then added a dab more solder, let it flow.
Usually that was quick. I had some track that I coudn't get to solder, I think it was just old, either switched to another track, or replaced the one that was being problematic.
One of the few things I learned about soldering (besides that I could never do it professionally, my dad was an EE who worked for a company doing defense work in the military aviation sphere, saw my soldering and gritted his teeth, haven't gotten much better *lol*), is that there are a lot of variables there, it could work great one day, not the next, same kind of wire, sometimes would solder great, not the other, also learned from a friend that there is value to replacing the soldering tip of the gun/pencil solderer (which being my father's son I am loathe to do), could also be the gun itself is not always heating right.
Only suggestion I have is vary the method you use when soldering, if you can, borrow a soldering gun/iron from someone and see if that works better, maybe the gun is marginal (and I am not saying that will help the original poster), use different solder/different rosin.
