You have me confused by having TIU in your title. You wouldn't use a TIU to activate crossing gates, you'd use an AIU or an isolated track section if using the train to activate them. Regardless, you can wire your transformer to a TIU or AIU and then the MTH DCS terminal block before wiring downstream (track or accessories).
Just google "MTH DCS Wiring Diagram" and you'll find just about every scenario.
-Greg