Here is a picture of my first fuel tender for my diesel fleet. It is a a tender from a Bachmann 4-8-4 with the drawbar replaced with an MT coupler. I looked at using tank cars, but decided that I would prbably run tinto the same problem tht the real railroad did. The y are too light for the buffering tht would result from being between two locomotives, so I have decided to use old steam tenders. They are larger and heavier and will work better.
More than a few railroads used steam locomotive tenders as movable fuel supplies - I recall seeing photos of those huge Pennsy long haul tenders in such service. I cannot recall seeing one used as a fuel tender for diesels in this way but it cannot be ruled out; it looks like something the resourceful Mexican railroads would cobble together.
I do have vivid memories (and some 35mm slides in some box or another) of the BN’s experiment with fuel tenders in long tank cars that were nicely painted in a BN scheme.