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Several comments, based on our design experience in TX. First, I agree with MC, Dave H etc and would strongly urge you to find another way to cross--the risks are just too high compared to the benefits (perceived or otherwise). That said, I would note that the examples cited by some of the folks involving dual use are for BIG bridges where one can successfully segregate the RR and non-RR uses, date from an earlier era when these issues were not considered to be quite as important, and thus are likely apples/oranges comparisons. Which leads me to a second point: under some unique circumstances you might be able to make something work IF you can very effectively completely ISOLATE AND PROTECT the pedestrian function from the RR, either by elevation, distance or by effective solid barrier (or some combination), and I do not mean a chain link fence or similar. You would have to protect from interference as well as from dragging equipment and loads shiftted if falling off the cars. Third, while you might be right about the additional pedestrian/bicycle (live) load on such an addition, it is not a foregone conclusion that the structural (dead) load of the addition itself will not overstress or reduce the load rating of the existing bridge. This issue is extremely important with the move to the 286,000 lb. weight limit and accompanying requirement to restress the bridge and piers (some of these can be pushed to the limit and will actually be overstressed in use, say by occasionally running a unit and a loaded 100 ton car very slowly over a 210,000 lb rated structure). Assuming that we all haven't talked you out of the idea, I would encourage you to (1) get the concept to the RR now, and pay extremely close attention to what they say, (2) if the RR will even consider the concept (which I really doubt, unless the government itself owns the railroad line and they're willing to take big risks) have a competent and experienced independent, licensed railroad bridge structural engineer (current Oregon P.E. license, in your case) with adequate Professional Liability insurance, do a very careful study of the structural issues and provide you a complete report and recommendations under his/her seal (if he/she isn't willing to seal it, it's not worth the paper it's printed on), and (3) get out your checkbook, because it likely is not going to be cheap. Probably the biggest problem that we have seen here on publicly funded RR projects is the erroneous idea (usually in the name of intermodalism) that you can freely mix pedestrians and, worse, some bicyclists (no offense, cyclists, but you know the types I'm referring to) with heavy railroad traffic, just because the trains don't run all that often. It doesn't matter how infrequent they are if some person rightfully using your facility gets whacked because they happen to be there at the "wrong" time. If the RR line is being abandoned, that's a different issue. Please give your idea extremely careful thought, consideration and defensible study before doing it. Safety is paramount. E. P. Hamilton III, Ph.D., P.E. (TX)
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