I tend to pour over time tables fairly frequently, and for the longest time I have wondered what it meant by "Bridge class engines and derricks...53.8", like in the 1970's era time tables. Could somebody please shed some light on that? Is it a weight issue? Do derricks refer to pile drivers? Codeman
The number refers to the Coopers live load rating for a bridge. Theodore Cooper, a civil engineer, developed a system to calculate the loading of engines and trains on bridge structures. The Cooper system is still used by railroads today. If you look at Frisco timetables, especially in the era of steam, you will find a Coopers rating for each engine. This would determine the maximum engine size that can be operated in a given branch or subdivision.
I share your interest in Employee Timetables. They contain tidbits that cover a broad spectrum of operations of a given subdivision at a given point in time. Like you, I was curious about the bridge class tables. Why did most of the mainlines possess a bridge class of E-70.4, and why did certain secondary lines possess a bridge class of 53.8, etc? Why the letter “E”? Clearly, the numbers have some relationship to the strength of the bridges, and the weight of the locomotives. At the time, my diesel-era only ETT collection did not provide much in the way of answers. The Cooper’s rating for the first generation diesel locomotives didn’t appear to correlate with the Bridge Class of the various subdivisions contained in the ETT’s. After I bought my first, steam-era ETT, the ol’ 40 Watt light bulb came on. There in the Bridge Class of Engines Table was the answer… …1306 Class, 2-8-0, E-53.8, 4500 Class, 4-8-4, E-70.4… It should have been obvious. The Cooper’s rating of the bridges was determined by the heaviest class of steam locomotive employed on a given line. Unclear to me was the method used to determine the Bridge Class of a steam locomotive. From time to time, “Model Railroader” would publish a schematic of two consolidations, coupled, and a table of varying axle weight and spacing, which defined a specific Cooper’s rating. However, the Model Railroader failed to discuss how the rating was calculated for locomotive of other wheel arrangements and other axle spacing. I have Grandfather Brand’s 20th Edition copy of Trautwine’s “The Civil Engineer’s Pocket-Book”. The first edition appeared in 1872. The 20th Edition brags that nearly 300 pages about railroads have been added. The words pocket-book are a true understatement, since the 1525 page, 4”x 6.5” book is 2 inches thick. It’s hardly a vest-pocket book or hip-pocket book. The remarks found in the 1872 edition’s Preface are fascinating, and I am certain that those with a “P.E.” after their name would take exception with the some of the comments. “Should experts in engineering complain that they do not find anything of interest in this volume, the writer would merely remind them that it was not his intention that they should. The book has been prepared for young members of the profession; and one of the leading objects has been to elucidate, in plain English, a few important elementary principles which the savants have enveloped in such a haze of mystery as to render pursuit hopeless to any but a confirmed mathematician. Comparatively few engineers are good mathematicians; and in the writer’s opinion, it is fortunate that such is the case; for nature rarely combines high mathematical talent, with that practical tact, and observation of outward things, so essential to a successful engineer…” Times certainly change! The Trautwine Pocket-Book provides an interesting look into the state of the railroading art in this case, 1922. Turning to page 755, one receives a succinct overview of the Cooper formula, and how to calculate the Cooper rating for other wheel arrangements and other axle spacing. See the attachments for the formula. For the modeler, the ETT’s Cooper’s ratings and the ETT’s Tonnage Ratings can be helpful with determining what steam locomotives might have been used on any given line. This is especially true when photographs are scarce. Check the some of the steam-era ETT's posted on this web site.
