No worries, I got yer back.
In the "Train - Pick furthest from (0,0)" event, I added, "Train - X is less than (scrollx + WindowWidth/2) + 100)". That intimidating expression, "scrollx + WindowWidth/2", just means the edge of the screen, since scrollx is the center and WindowWidth/2 is, well, half of the window's width!
So if the last train is within 100 pixels of the edge of the screen (but still OFF screen), create the new train car. Seems to work right now, but like I said, it might not work once it gets faster and faster. If it doesn't, just change that "+100" to something bigger, like 500.