Trains should burn low-grade fuel first

[Ardagan]

Currently:
you can't clearly predict which type of fuel train will burn. There should be logic, but hard to deduce.

Proposed:
Train should burn lowest-grade fuel first.

Use case:
Convenient way of clearing your locomotives of old types of fuel and replacing with brand-new standard.

[Koub]

Or they could always use the higher grade, to be as quick as possible given the fuel options.
There is no "better" rule, just a matter of personal preference.

[Zavian]

Ah doesn't it already have a simple and logical order to the fuel it burns?

If I recall correctly a train burns whatever is in the first fuel slot. Whenever that runs out, the contents of the second slot move to the first, (and the third slot moves to the second). That way you can start your trains off with wood or coal, then later upgrade the fuel source to solid or rocket/nuclear fuel, and the trains will use whatever older fuel they have, then switch to the new fuel.

[Darinth]

I'd always assumed that it worked the way Zavian described... but I have to acknowledge that I never bother mixing fuel in my trains. I've always pulled the old fuel out when I was ready for an upgrade and used it other locations.

[Byproduct]

I could be wrong but I think if you just start feeding the new fuel type to your refueling stations, the old fuel gets used eventually and it doesn’t take too long for that to happen. I don’t recall running into an old train and noticing ”oh there’s still coal in here” or anything like that.