As you can see, the reflections impair the visibility of the objects behind the glass quite a lot. This image is fairly old (2009), so that I can't revisit the site for a better shot.
Software: darktable + Gimp 2.8.
Although I am not a Gimp User, but the process I use in Photoshop will most likely be very similar and it should not take more than 5 minutes to reach a satisfactory result.
I will start with the Clock first
Now for the Platform Indicator
Create a Curve Adjustment Layer, but DON'T do anything just yet.
Then from the COLOUR SELECT, use the dropper tool to Select the areas you want reduced (The Reflections area)
This will now allow the Curves Adjustment to only effect this.
Pull down the curve adjustment until it matches. ( you may have to do several separate passes to achieve this.
Unfortunately, this will effect some of the writing too ( Wilhemshagen) and for this, you need to either.....
...... Clone in the letters from other more readable areas
...... Or, create COPY VIA LAYER for each letter required and bring them down to create Wilhelmshagen.
With the “W”, you will need to draw that in as there isn’t one to copy from
Finally, just clone and tidy up
I could come up with the following using Gimp:
As you can see, I only removed the reflection at the word "Erkner". You would probably get better results using a RAW version of the photo and by spending some more time.
Here is what I did:
Credits should go to this video (he uses Photoshop, but the process is the samein Gimp).