The default german Class 143 has some wrong values in it's blueprint files. RSC knows about it and has the correct values for a long time now, but somehow the core programme was never patched. The version of the payware addon of Class 143 has the correct values in it.
The loco in Railworks can only reach a speed of about 112 kmh for two reasons: the brake is never fully released and the value of maximum speed is wrong. Here is how you can correct both things in the engine's blueprint. Please note that any Steam update of Railworks may revert the changes to the default file, so keep a backup of both versions.
First find this file in your Railworks folder:
...\Assets\GermanRailroadsRW\Rollmaterial\Elloks\BR143-CabSimAudio\Simulation\BR143 Engine sim.bin
Make a copy of it, so you have a backup just in case something goes wrong. Now open the file to edit it. The easiest way would be to use RWTools, but you can also do it manually.
Copy the file into the main RW folder and drag & drop it onto the "serz.exe" file. This will convert the .bin file to an .xml file you can edit with any text editor, I would recommend Notepad++, but the windows notepad is also fine. Just don't use Wordpad or MS Word or anything similar.
When you have the file opened look for this section, it is approximately in the middle and behind a section named <Handbrake>.
<GraduatedRelease d:type="cDeltaString">eTrue</GraduatedRelease>
<ProportionalBrake d:type="cDeltaString">eTrue</ProportionalBrake>
<MaxReleaseRate d:type="sFloat32" d:alt_encoding="0000000000002440" d:precision="string">10.0000</MaxReleaseRate>
<MaxApplicationRate d:type="sFloat32" d:alt_encoding="0000000000002440" d:precision="string">10.0000</MaxApplicationRate>
<MaxCylinderPressure d:type="sFloat32" d:alt_encoding="0000000000805640" d:precision="string">90.0000</MaxCylinderPressure>
<PressureForMaxForce d:type="sFloat32" d:alt_encoding="0000000000005440" d:precision="string">70.0000</PressureForMaxForce>
<MaxSystemPressure d:type="sFloat32" d:alt_encoding="0000000000005440" d:precision="string">70.0000</MaxSystemPressure>
<MinSystemPressure d:type="sFloat32" d:alt_encoding="0000000000000000" d:precision="string">0.0000</MinSystemPressure>
Change these two values to 80.0000 so that the brake will be release completely.
Further down you also want to adjust the maximum speed of the loco:
<MaxSpeed d:type="sFloat32" d:alt_encoding="0000000000E05240" d:precision="string">74.000</MaxSpeed>
Change it to 75.500 which is about 120kmh. The value here is given in mph.
Save all your changes and if you edited the manual way, convert the .xml file back to .bin with serz.exe and copy the new .bin file back into the correct folder. Keep a backup of both versions!