[Bug]: unpowered trailing parts of articulated engines use pax company colour

[andythenorth]

Version of OpenTTD

12.2

Expected result

Trailing parts of articulated engines use company colour set by player for engine class.

Actual result

Trailing parts of articulated engines use company colour set by player for passenger coach (engine class).

Steps to reproduce

Using Iron Horse 2.61.1

Set company colours per the screenshot.

Observe that trailing parts of engines 1, 2, 6, 7 are using the pax coach colours. See note about engine 8 further down.

Issue does not occur in Iron Horse 2.48.0.
Bisecting shows that the issue was introduced by the following commit andythenorth/iron-horse@aebd97a

That commit brings Iron Horse into compliance with the grf spec, where power for trailing parts must be 0 (zero).
https://newgrf-specs.tt-wiki.net/wiki/Action0/Vehicles/Trains#Properties

If the power for trailing parts is non-zero, the company colour bug is not present, however that is not compliant with the grf spec. Note that setting power on trailing parts does not appear to affect the actual power used in game, which seems to depend only on lead part (although the callback may also be affecting this result).

In the screenshot, engine 8 is using the correct colour as it has a different implementation, and has power set (incorrectly) on the trailing part.

Options to solve this appear to be:

• change the grf spec
• change OpenTTD's implementation of articulated vehicle colour selection
• call it a feature?
• something else?

[frosch123]

The "expected behavior" is the intended behavior.
Code is here: https://github.com/OpenTTD/OpenTTD/blob/master/src/vehicle.cpp#L1892
No idea why it wouldn't work.

Oh, and the e->u.rail.railveh_type != RAILVEH_WAGON part checks for power != 0.
Not sure what this wagon-exception is about.

[JGRennison]

https://github.com/OpenTTD/OpenTTD/blob/master/src/vehicle.cpp#L1890
(v->IsArticulatedPart() && e->u.rail.railveh_type != RAILVEH_WAGON)

Setting the power of the trailing parts to zero sets railveh_type to RAILVEH_WAGON

[frosch123]

For futher reference: 336fda6