What's the rule of thumb for those times where you lack the right photographs and have no drawings to go off of?
I'm starting on my high line for my steel mill complex. I have photos to go off of for the areas where there is space for a big concrete support but there are tight spots where the concrete supports won't fit (mostly because of crossing tracks at a weird angle). I haven't encountered this for this model yet because I have at least had something to go off of for everything up to this point.
What do you do when you have to model something from the past but have nothing to go off of? Is anybody here an expert in industrial concrete/steel trestles from the 1920s? 
My current backup plan is to study some photos of Pittsburgh mills and also the train in Chicago that runs above the street in the Blues Brothers.
Well, I suspect the short answer is that it depends what the ultimate goal is for you.
If the primary objective is to produce as close to historically accurate depiction of an actual structure, or at least closely emulate the style and features of a particular era or school of construction, then yes, you need to pursue all available research paths until you are satisfied you've exhausted all possible sources, or at least enough to have a clear path forward. Once you have that clear vision, you can extrapolate as needed to "fill in the blanks", and deal with all the practical considerations, such as limited space or available materials, that might advise/require compromises.
OTOH, if your goal is to just evoke the area or type of construction you set out to model, you are entirely free to stop your research at any point you are satisfied you have a suitable way forward. Let's face it -- for most of us, we're building our layouts primarily to satisfy an audience of just one, so IMHO it's a matter of how you want to best occupy your time. If you really enjoy researching such issues, knock yourself out! OTOH, if you're more into the construction process and enjoying the fruits of your efforts, I suspect "good enough" would be a better guiding star. Good luck, in any event! 
