At least for German signalling rules, these sites are trying to explain it in English: http://www.sh1.org/eisenbahn/index.htm http://www.sh1.org/eisenbahn/index

At least for German signalling rules, these sites are trying to explain it in English: http://www.sh1.org/eisenbahn/index.htm http://www.sh1.org/eisenbahn/index.htm or http://www.joernpachl.de/German_principles.htm http://www.joernpachl.de/German_principles.htm (Railway Engineering professor)