This hasn't happened to me but I observed falls when:
1) People try to cross RR crossings at any angle other than 90 degrees (That is to say you should cross it like you're crossing a T.)
2) People trying to ride across wet RR crossings. My opinion is get off and walk your bike. Those things are slicker than ice.
One other thing: Do what road workers do. Wear high-visibility colors (or colours if you're in the UK).
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"Light it up, Popo." --Levi Leipheimer