Oooops!
Went back and read the first few comments of this thread.
It appears that Generic Counters are preferred.
So... Generic it will be, now for the color?
Blue counters are generally water, but generic transport are mostly white (Skis, Horses, Wagons, etc.)
I'd propose generic counters for generic units. There might be some armored trains/wagons that are specific to a certain nationality (The Germans for example used armored trains made up of individual self driven wagons of different specifications that were linked up to form a train.). In such cases, the counters should be in the nationality color.
As the situation in which
two armored trains battle with each other of opposing nationalities will probably never come up, no harm is done when mixing generic armored train counters with some specific nationality counter colored ones if need be. And even in the extremely unlikely case of an 'armored train' duel, still nobody would confuse the
two trains..
Generic counters are usually black on white (Wagons, Sidecars, Motorcycles, Bulldozers, Skis, Mines, Wire, Parachutes, Horses, ...) as you have pointed out. So this would be my proposal for generic armored train counters as well.
von Marwitz