I Know I am waiting to put them there because I had them there before when I airbrush the track they blow out, I am going to wait then paint them and slide them under. MikeLooking good Mike. As Jim pointed out make sure you are absolutely leak proof before you pour.
I noticed in your third pic there is a joint in the flex track. If you take a couple ties, shave off the spike detail and sand the bottoms they will slide right in under the joiners and look better when you ballast.