It's called research; you know - looking things up in books and online. Are you really trying to reproduce the prototype without even a photo? Good luck with that!
Modelling isn't just buying RTR models and, perhaps, putting new transfers on them. Those of us who do the job properly have probably spent as much, if not more, on reference books than we have on the models themselves.
This forum is an excellent source for information, but it's unreasonable to expect those who buy reference books to do research for those who don't.
And for many of us that is a big part of the fun. Even after I complete a model, if its something really unusual, I'll occasionally do a quick Google search to see if anything new has appeared to fill in any gaps. But sometimes you have to accept an educated guess. An example would be my Tunnel Inspection Coach DW150144. I figured if I couldn't find the info I wanted, it possibly didn't exist. But, my model was based on the old Hornby coach, and they have now done them as a modern super detail version (and the correct hand, which'll save a lot of cutting and shutting) so things that have come to light since the first model will go towards the second.
A mate of mine will often call be to check on details of things, once he has exhausted his references. Not because I'm an expert, but because I'm slightly older than him and therefore more active, and with a camera, in his chosen time period. We were trying to figure out roof mods and boiler blanking plates on a particular class 47 just last Sunday. No definitive conclusion on that one, but our combined efforts uncovered a few other relevant facts.
Said mate works on the railway and when he was learning HST stock the instructor, himself an acknowledged expert on the subject, was surprised at the knowledge he already had of the subject. "Of course, I'm a railway modeller", came the response!
OK, we've strayed from rail tank cars, but I felt John's comments deserved more than an "Agree".