Is it possible to use a LEGO NXT ultrasonic sensor for a LEGO train crossing?
As you suspected, you cannot use the Ultrasonic sensor without an "Intelligent Brick" (EV3 or NXT). MINDSTROMS would be a great way to automate your train if you are willing to make the investment.

For more ideas on how to use LEGO MINDSTORMS with LEGO trains, check out Sioux.NET.