If you connect a tank car filled with diesel to a diesel locomotive, the diesel locomotive is not filled by the tank car.

Diesel fuel tenders are used in real life, and I think Traincraft should also allow for such a configuration:

No new models or textures need to be created for this feature.

In real life, those fuel tenders are used to save money by buying it cheaply somewhere and using it to save money by extending the range of the locomotives far into the other states.

But then again, you should be able to specify whether it's a tender or a regular vehicle.

Maybe shift-click to toggle tender on/off, like the locomotives toggle pull/be pulled?

That is an interesting concept....

I may add a GUI button to toggle tender status for tanker cars in the rewrite.
Theoretically that could even allow for multiple tenders. Alpha 3 will be the time for tanker cars so its a good point for me to think this kinda stuff over.
speaking of I'm also gonna label this one rewrite, because i doubt anyone wants to do this to the port (for various reasons).

Say, I came up with an interesting idea, how about if it carries diesel, it's a tender for a diesel locomotive, if it's filled with water, it would be similar to a water tender for steam trains.

Kind of uncommon to see a water tanker hooked up to a steam engine's boiler.
Besides that the current tenders for steam already do that, and then some.
That does remind me i need to overhaul the fuel systems in the rewrite. I keep forgetting that...

It is uncommon, but for example the 'Flying Scotsman' was fitted with dual tenders due to the fact that water was scarce to obtain, the first tender only held coal while the second tender held water, I feel that travelling HUGE distances in Minecraft using a steam locomotive in a place like the nether would be difficult without additional tenders.

Well yes, but that was dual tenders like what already exists in TC, just dual. Not using a water tanker on a steam locomotive.
I mean I'm not fully against the idea, I'm just saying its kind of redundant to add a car for just water when current tenders already carry water and fuel.

Then again.... In the case of a rewrite, a tender for a nuclear steam train would only need water/coolant, which a nuclear steam train would need a lot of.
hm.....

What dual tendered locomotives already exist in TC?

I'm sorry, my point was poorly stated... What I meant was, in the TC port, all tenders have ability to hold water and burnables.
I agree that support for hauling multiple tenders should be added, if nothing else in the rewrite, but, when most tenders in TC support water and burnables, something that will continue in the future, I think it's a bit redundant to have steam trains be able to get water from water tankers.
With the exception of nuclear steam, which I would think work that way normally.

I don't think it's irrelevant for steam locos to pull water from tank cars for a few reasons.

1. Tank cars carry the largest volume
2. You can't bank the fire to produce less steam
3. AFAIK water consumption is not tied to velocity, which is how it should be (sure, there's a little burnoff while stationary but not a lot)
4. There is no auxiliary tender model.

True on 1 and 2.
3, It tries to be, but it never worked.
And I have no idea what you're talking about on 4.

At any rate, since the idea does seem to be rather popular, I'll do something about it in the rewrite, I'd rather not have to deal with another tender crash issue in the port, lol.