Why no tram lines in Sassoon fonts?

The reasons are discussed in the books by Rosemary Sassoon.
Briefly, each child has a slightly different size of hand which affects the comfortable range of handwriting movement. Providing exact tram lines pressures the child into modifying this comfortable range and whilst it may initially help, very quickly, the child will resort to what is most comfortable. The result will be that handwriting will overshoot tram lines that are too close together, while tram lines too far apart will be ignored.

It should be noted that the PDF Copybook for the separate lettered fonts (letters that do not yet join), we provide a solid line for the baseline and a band of colour suggesting the x-height of letters - it is not a line. For this reason we have created Sassoon Primary Infant Line.

For the PDF Copybook of joining letters, a baseline is all that is required.