double level crossing for OO track, just fit code 100 rail through slots in end, like the tramway /inset track. Requires gates, gear mechanism pack, and control arm. Fit rail before painting, as it tends to be easier. Can be a very tight fit, so suggest running lenth of thinner rail(code 75) through first. Slow and steady is better and safer than rushing it.
When fitting gear wheels, take it slowly, they do fit, and when in place , spin round till nice and smooth running.