[Csantucci]

Even if ORTS emulates well the MSTS behaviour of trainsets and the related sound and cabviews, there will always be cases where it would be better to modify .eng, .sms or .cvf files to get a better ORTS behaviour. Also using functions introduced by ORTS sometimes requires to have ORTS specific versions of such files.
The problem of course is that, if you modify such files, also the MSTS behaviour could be modified.
BUT... there is the solution: the description of the solution is only rapidly outlined in the ORTS manual, so I think it may be useful for others if I expand a bit what I could successfully try (also thanks to the hint of gpz).

The solution is creating, within the folder of the trainset you want to modify for ORTS, a folder named openrails. Within this folder you put your .eng file modified for ORTS (with the same name of the original file), and here you are! ORTS will use such file instead of using the original file. In this file you can e.g. change braking parameters, or refer to different .sms and .cvf files.
The wise OR developers put at your disposal a further useful feature. If you start your .eng file this way

include ( ../FS_ALn668-1709.eng )

(where the first empty line must be there) you can put in the rest of the file only the .eng lines you want to modify, provided you respect the .eng file structure.
Better than words I put here a complete example:

include ( ../FS_ALn663-1112.eng )

Wagon (
Inside (
Sound (  "../../Common.snd/Italy loco sound/ORTSFS_Aln663cab.sms" )
)
Sound ( "../../Common.snd/Italy loco sound/ORTSFS_Aln663eng.sms" )
)
Engine (
	AirBrakesMainMaxAirPressure( 120 )
	AirBrakesMainResVolume( 24 )
	AirBrakesCompressorRestartPressure( 110 )
	TrainBrakesControllerMaxSystemPressure( 75 )
	TrainBrakesControllerFullServicePressureDrop( 64 )
EngineControllers (
        Throttle ( 0 1 0.025 0
            NumNotches ( 6 
		    Notch ( 0.000   0 Dummy )
                Notch ( 0.20   0 Dummy )
                Notch ( 0.40   0 Dummy )
                Notch ( 0.60   0 Dummy )
                Notch ( 0.80   0 Dummy )
                Notch ( 1.000   0 Dummy )
		)
        )
)
Sound ( "../../Common.snd/Italy loco sound/ORTSFS_Aln663cab.sms" )
)

With this file I modified following things with respect to MSTS operation:
- .sms files
- some braking parameters
- and replaced a continuous throttle with a notched throttle, because this is more prototypical.
Notice how the structure of the .eng file must be respected (wagon related parameters are within the wagon () section and so on).

What was found out by others is that, if you want to modify Lights () parameters, you must insert the full Lights() section of the .eng file. I didn't test it.

The very important thing is that this way no file read by MSTS is modified, and so MSTS operation is fully unaffected.
This also would allow to use an extended keyword set (understandable only by ORTS, and that could also lead to MSTS errors if the latter would read such file - which does not happen because the file is in a directory not accessed by MSTS) for .eng, .sms and .cvf files. I hope that the ORTS steering team will take this into account and allow for extending such keyword set where desirable.

[James Ross]

Mmm, I'm not sure if that feature (the OpenRails folder) is still in the code, though it certainly was. We should probably decide which files would benefit most from this and cleanly enable it only for those files.

[Csantucci]

The feature is there (release 2110). I have just made available in my country a set of files like the one I have inserted in my post. Pls. don't disable it, it's a genial idea! I neither see the need to partially disable this feature.

[James Ross]

Csantucci, on 20 March 2014 - 07:45 AM, said:

The feature is there (release 2110). I have just made available in my country a set of files like the one I have inserted in my post. Pls. don't disable it, it's a genial idea! I neither see the need to partially disable this feature.

I don't think the idea was agreed before being added, so we might limit which files it applies to. However, I'd say that engine/wagon files have a pretty strong case for being split. We do need to balance complexity of such features with usefulness and with keeping content well organised.

[Eldorado.Railroad]

If this does what I think it does it will solve the dilemma of how smoke effects are handled differently in OR. This means I can leave the original MSTS files alone and instead create a new file with the preferred values for OR. This feature gets my vote, I will try it later today. Let's keep it please. Thanks Carlo for bringing this to the OR users attention!