This answers might help: "The numbers refer to the frog angle. The higher the number, the smaller the angle and the longer the turnout. A number 6, for example, turnout is measured by 6 units from the point of the frog to where 1 unit between the diverging rails occur. A unit can be an inch, cm, or whatever. Again, the higher the number, the "broader" the turnout. A number 6 is considered the minimun for satisfactory mainline (model) performance IMHO. In the prototype, we find frog numbers of 20 and up."
"The "number" of a turnout is the ratio of its divergance.
In other words, with a # 6 switch the tracks diverge 1" in 6 inches of distance. With a #4 switch they diverge 1' in 4' of run. The lower the number the 'sharper" the switch. Rule of thumb is that 4 axle engines and 40 ft cars will go through a #4 switch ok. 6 axle engines and long cars go through a #6 switch ok. Everything goes through a #8.
By the way a typical prototype yard switch is a #8, a typical industry switch on the main is a #10, a typical siding switch is a #14 or #15 and a higher speed mainline switch is a #20 or #24 switch. A #20 crossover is over 4 feet long in HO."