MI-Marc,
I like to keep my TIU's just above the 1st level of my platform, when I do this I never have any signal problems, test as you build your layout, use the 10 - 12 join technique when laying your track. I had 5 levels to my FasTrack layout and it ran perfectly, I did use some magic lights to attain all 10's thru out my multi level layout.
I eventually eliminated the AIU's and went with Command Control FT switches, on my Christmas layouts. In this manner I got rid of more than 50% of my wiring.
PCRR/Dave