there is a page on wikipedia about how that works, but the page refers to signals to tell if the section is occupied, find that one, when the signal shows 'do not proceed', replace that with the flashing lights and bells. i've done axle resistance measuring before, and it's usually around 3-6 ohms, if that helps. usually, the is a fair amount of track before the crossing on both the up and down sides, but if there is a station on one side of the crossing, thats different.