First of all You need to locate the res folder in the Nordic DLC folder which is usually found in "Steam\steamapps\common\Train Fever\dlcs\nordic_1".
In this folder there are a number of subfolders which You will need to recreate in Your Transport Fever directory as well.
The models have a predetermined path, so You must use the exact same folder structure in Your Transport Fever folder as the one used in the Nordic_1 folder, otherwise the game will likely CTD when You buy any of the vehicles provided that they show up at all.
For vehicles it is a little complicated, but not impossible since most folders already exist in Transport Fever, so we will basically tell the game that the Nordic DLC vehicles are included as vanilla vehicles in terms of folder structure at least.
This is how You import locomotives:
1. open the models folder in Your Train Fever folder (usually "Steam\steamapps\common\Train Fever\dlcs\nordic_1\models").
2. open the models folder in Your Transport Fever folder (usually "Steam\steamapps\common\Transport Fever\res\models").
Now compare the subfolders in both folders, they should look roughly the same so far.
3. open the model subfolder in both models folders. ("Steam\steamapps\common\Train Fever\dlcs\nordic_1\models\model" and "Steam\steamapps\common\Transport Fever\res\models\model" respectively).
4. open the vehicle subfolder in both model folders.
5. open the train subfolder in the model folders ("Steam\steamapps\common\Train Fever\dlcs\nordic_windows\nordic_1\res\models\model\vehicle\train" and "Steam\steamapps\common\Transport Fever\res\models\model\vehicle\train" respectively).
6. Now You see a list of all the trainmodels available in the Nordic DLC (they should end with .mdl), so just copy all or the ones You want from the Train Fever train folder to the corresponding Transport Fever train folder.
7. Once You have pasted all the modelfiles You want in the train folder, You need to back up to the models folder again.
8. Open the mesh subfolder in both Train Fever and Transport Fever folders ("D:\Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\models\mesh" and "Steam\steamapps\common\Transport Fever\res\models\mesh" respectively).
9. Open the vehicle subfolder in the mesh folder in both folders.
10. Open the train subfolder in the vehicle folder ("Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\models\mesh\vehicle\train" and "Steam\steamapps\common\Transport Fever\res\models\mesh\vehicle\train" respectively).
Now You should see a number of subfolders representing various models.
Now comes an important part as some locomotives share dependencies such as boggies an pantographs, so You should always copy those folders regardless of which locomotive You want to use.
Now copy the folders representing the locomotives You want to use in Transport Fever from the Nordic DLC folder and paste them in the Transport fever folder.
Once You have the model's folders You want including the pantograph folders if You want to use the electric locomotives You can back up to the models folders in both directories again.
11. Open the material folder in both directories (usually "Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\models\material" and "D:\Steam\steamapps\common\Transport Fever\res\models\material" respectively).
12. Open the vehicle subfolder and then the train subfolder in both directories (usually "D:\Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\models\material" and "Steam\steamapps\common\Transport Fever\res\models\material\vehicle\train" respectively).
Now You will see a list of material files (should end with .mtrl) representing the various locmotives in the DLC.
Copy all or the files You want from the Train Fever Nordic DLC folder to the corresponding folder in Your Transport Fever directory and don't forget the pantograph files if You want to use the electric locomotives.
13. Back up to the res folder again in both directories ("Steam\steamapps\common\Train Fever\dlcs\nordic_1\res" and "Steam\steamapps\common\Transport Fever\res" respectively).
14. If You copied the SJ Y1 and Y6 files in the earlier steps You will now need to copy the corresponding config files as well (only needed for Multiple units).
To do this You open the config folder in both Your Trainfever and Transport Fever directories (usually "Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\config" and "Steam\steamapps\common\Transport Fever\res\config") and open the multiple_unit subfolder.
15. Once in the multiple_unit subfolder You should copy the files from the Train Fever folder (usually "Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\config\multiple_unit")corresponding to the models You copied earlier, but only those since the game will likely crash upon starting or loading a game if You have config files not corresponding to the same model files.
Now paste the files You copied into the corresponding Transport Fever folder (usually "Steam\steamapps\common\Transport Fever\res\config\multiple_unit") and then back up to the res folder again.
16. Open the texture subfolder in both directories (Usually "Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\textures" and "Steam\steamapps\common\Transport Fever\res\textures") and open the models subfolder.
17. Open the vehicle subfolder and then the train subfolder in both directories (usually "D:\Steam\steamapps\common\Train Fever\dlcs\nordic_1\res\textures\models\vehicle\train" and "Steam\steamapps\common\Transport Fever\res\textures\models\vehicle\train" respectively).
18. Copy the files You want from the Train Fever folder to the corresponding Transport fever folder and again don't forget the pantograph files if You want to use electric locomotives.
Now You should have everything You need provided that You also have Merk's conversion mod installed, so start the game and go to a train depot and see if You have the locomotives available to purchase.
The Train Fever locomotives will not have any miniatures/thumbnails representing them in the train depot, so You will have to read the description behind the blue box to see which locomotive it is.
It is highly recommended to save the game before trying to buy any of the imported locomotives or multiple units for the first time.
This way You will not lose any of Your progress if a dependancy is missing.
IF a dependancy file is missing the game will CTD, but it will luckily also tell You which file is missing and the path to where it should be.
If this happens You will just have to locate the needed file in the Train Fever Nordic DLC folder and copy it to the corresponding location in Your Transpor Fever folder and try again.