I read it in a book somewhere, I think a CJFreezer tome. It seemed authoritative at the time. (196... :-)
"Continental Europe" is a big place, with many different track standards and I would hesitate to pick any one track configuration that would represent the total. Flat bottomed rail might be one commonality over most of Europe - regularly spaced sleepers another, but that's about it!
Regards, Greg.P.