I just realised there is a comment here.

So, after over a year of wondering, I am here with the answer:
Luck?
And a little engineering. Mostly the trick is to get a solid gauge all the way through the curve and then just try to match up the rail ends. Remember that these trains have super short wheelbases so my very rough tracks won't harm them that much.

how did you manage to make the trains curve? this has been a problem for me forever!