It’s extended.
In that case, you can find the
Linux makeobj-extended here and
the Windows .exe here. Place this in the same directory as the .dat file.
Simutrans-Extended normally expects the images to be in a subdirectory. For example, the Class 156 images are in the 'railcars' subdirectory:

So put the .png files in the subdirectory specified in your .dat file. Then open a command prompt (in Windows Explorer, shift+right click in an empty area of the folder) or terminal (Linux) in the main directory and run
makeobj-extended pak128
The .pak file should appear in that directory. So the end result should look something like this:

P.S. James, could you consider amending Step 5 of your otherwise excellent
Step-by-step Tutorial along these lines? Recompiling the whole pakset is a very sensible way for you to run the Bridgewater-Brunel server, but it requires scripting Python (and possible Make?), which just adds an extra layer of considerable complexity for us newbies (who are struggling with Blender and .dat files at the same time). And of course it slows down the process (well, unless you are lucky enough to have a Threadripper and an SSD

). What we need is to be able to see any makeobj error messages that relate to the pak we are working on.