These were introduced because often “… the bearing surfaces at the ends of stringers and caps in railroad trestles (were often) too small.” In the end objections to their use was variously – increased cost, increasing timber against timber which increased the liability of decay – a somewhat interesting argument but over a century old. Ultimately, they were a stop-gap .. better selection/design of the stringers and caps made them obsolete since their use stopped.
I can make use of them though, in my trestle. The trestle has about 2/3 of it’s length on a curve .. consisting of approximately 3″ long segments. Where the join is a probable week point. Corbels can reinforce these joints .. and what the heck .. they look cool tool.
There’s no set size. –
For a 7-3/4″ x 15″ x 14′ stringer the corbel was 7-1/4″ x 9″ x 2′
For a 8″ x 16″ stringer the corbel was 8″ x 15-3/4″ x 4′
 Trestle Superstructure Corbels. – In both frame and pile trestles the superstructure consists of everything above the cap, viz., corbels, stringers, cross-ties, and guard timbers. The first sketch in Fig. 176 represents a side view of the superstructure at the bent, and the second sketch shows a longitudinal view of part of the superstrucure, aa being the ap.
The corbels, or bolsters, bb for two 8 xby 16-inch stirngers, consist of a block 8 inches thick, 16-3/4 inches wide, and 4 feet long, resting symmetrically upon the cap aa and supporting the stirngers cc. The corbels are notche 1 inch over the cap, and should be drift-bolted to the cap; the drift-bolts have countersunk heads.
 Economical designing of timber trestle bridges – A.L. Johnson – 1902
 Elements of railroad track and construction – Winter Lincoln Wilson – 1915
Note: Bolts run through the corbel up through the stringer and through the ties .. one on either side of the rail. The corbel is the width from the outside on one stringer to the outside of the other.
My stringers are similar to those in Fig 76 being a pair. the width is .270″ or about 13″ in width. I have some strip-wood measuring 3/16″ x 1/4″ – equating to a 9″ x 12″ corbel. That should suffice. I need a total of 10 corbels, each 1″ (4ft) long and notched 1″ or so.
In the end I didn’t use corbels. I had actually glued some on .. and decided that the trestle just curves too much .. the corbels just made that apparent (more so) – and then looking closer I realized that because of the angle of the lower track that some of the bents have to be placed an angle .. and therefore those corbels would hang in the air. Oh well.