Some resources ..
The transition spiral, earthwork, railroad location, trestles, trackwork .. etc etc. copyright 1907 ..
TRESTLES
Railroad Construction Theory and Practice pub 1914 -
Trestles
A treatise on wooden trestle bridges according to the present practice of American Railroads - pub 1904 - (
page 60 .. it says "Considerable economy may be effected in trestles of great height by spacing the bents further apart, say thirty feet, and supporting the floor on a deck truss."
Civil Engineer's pocket book : A reference for engineers pub 1918 -
on page 790 it says ".. The bents of wooden trestles are spaced usually from 12 to 16 ft. i the ordinary spacing being, perhaps. 15 ft., calling for 30-ft. strikers* are snipped conveniently on the average flat car ...
Railroad Construction - pub 1913 ..
on page 228 it says "Wooden Trestles with Ballast Floors. — There are two general types, one having the stringers separated and covered ... The spacing for bents is from 12 to 14 ft. centers. They first came into use in 1878 and no road reports having found ..."