if you have a friend who works for the railroad you're interested in, you might try to get a peek at their book of rules and most current issue of special instructions, as these will give you that railroad's standards for speedboards, their form and meaning.
not only do these vary in their nature and design from one country to another, but in some places, particularly north america, from one railroad to another, sometimes even one division to another on the same railroad.
i could quote you rule 10-j from the southern pacific book of rules, revised may 1, 1955, but i'm guessing that's probably not exactly what you're looking for, and the answer already given more likely is.
before that answer existed there was, to my knowledge, no uniform absolute standard, only voluntary standards of each railway as to what it considered safe. again as layed out in its book of rules.
