I claim the ornamental design for a trailer side rail, as shown and described.
FIG. 1 is a top perspective view of the trailer side rail showing a groove pattern on the exterior side wall.
FIG. 2 is bottom perspective view of the trailer side rail showing a groove pattern on the exterior side wall.
FIG. 3 is a left side view of the trailer side rail showing a groove pattern on the exterior side walls (the right side view being identical to the left side view).
FIG. 4 is a top view of the trailer side rail.
FIG. 5 is a bottom view of the trailer side rail; and,
FIG. 6 is a front view of the trailer side rail showing a groove pattern on the side walls (the rear view being identical to the front view).