DSP&P (UP) records of 1886 had it at 12.5x26.5.
DL&G records of the early 1890s had it at 12x26.
Do you have access to the 1917 ICC Inventory, specifically the Engineering Notes/Report? I don't but suspect that that would possibly provide more details on construction.
The problem right now is that the Archives in VA are closed due to the pandemic, for the foreseeable future. Perhaps someone on the forum (Rick Steele?) has the Report and can share that page(s).
Dave Eggleston
Seattle, WA