In some countries (Romania) they are -- you have to slow to a crawl since they are usually very bumpy. But I agree Waze is too aggressive, since train crossings are always well signaled.
I have been stuck on one way roads at a train crossing for 30 minutes at a time, making me way late for work. So I would actually love that in google maps.
Also why does Waze think train crossings are a huge deal. It seems to go to great lengths to warn me a bout them.