SDog,
thank you for your work on this. If you want to do a diesel locomotive, might I suggest either the class 47 or the 253 (otherwise known as the "class 43" or "Intercity 125" power car - not to be confused with the earlier class 43 Warship class) in British Rail blue.
I think that the symbol idea is better, however: steam could be represented with a smoking chimney, electric with the flash, biological with a horse, sail with a sail, diesel with a jerry can or oil drum, hydrogen with a cloud of vapour, and maglev with a magnet.
The Hood,
there is no easy way of specifying a maximum vehicle size for depots (and that would in any event require a change in the code). Shipyards are not constrained by space in the game significantly more than other types of depots as far as I am aware, or were you referring to the prototype?
The idea of traction types is more about where vehicles can be built rather than maintained, which is, after all, the function of depots in Simutrans. This distinction matches the prototype, in which facilities for constructing vehicles and conducting major overhauls are specific to their traction types. For those places where the two are combined on the same site - there is nothing to stop a user putting two depots next to each other, which would make the depots more realistic in size in any event.
The function behind traction types in economic terms is to give players a certain level of investment in the infrastructure necessary to set up particular types of traction, reflecting the cost of switching between different traction types.