The path is not fake, it is a real existing tunnel called tunel ,,Brđani" under Brđani hill, (Brđani means 'Hill' in english) and was part of first 5km (built in 1914) narrow gauge 760mm railway towards Loznica, that was stoped beacause of WW1 and WW2 respectively. The project was canceled and a standard gauge 1435mm construction commenced in 1998 southward, from this point. (Because, until the late 60s, Vladike Nikolaja south carrigeway used to be the a narrowgauge railway from Belgrade, terminating at Valjevo old railway station, (at Vladike Nikolaja 7)). 
Now for the facts:
Not fictional mapping. (For the standard gauge i used Geosrbija katastar, that has it parcelised and you can see the trace of 1435mm railway & here you have deatailed katastral numbers (https://otvorenavlada.rs/uredba-pruga-valjevo-loznica037-lat-doc/), (now i know there was a hacking problem and since then, the Geosrbija platform & Katastar are unavailable outside Serbia).

Note: It is not as precize, because the plan documentation, is still not available, but i took the refferences from GUP-PDR Loznica & Osečina. For the Valjevo-Osečina part, it is relatively visible on the satellite imagery.

Now for the above questioned obstructed tunnel path from the old narrow gauge addition, please see the following: 

Eastern portal here: (https://www.google.com/maps/@44.2738523,19.8774945,3a,29y,222.57h,85.33t/data=!3m6!1e1!3m4!1sSz6mQRrT71V-zBKfr9v2Gg!2e0!7i13312!8i6656?coh=205409&entry=ttu&g_ep=EgoyMDI0MDkwNC4wIKXMDSoASAFQAw%3D%3D)

Western portal here: (https://www.google.com/maps/@44.2715789,19.869157,3a,56.9y,69.67h,94.26t/data=!3m6!1e1!3m4!1sBqUb3OPYbTdPIkwI78cTtA!2e0!7i13312!8i6656?coh=205409&entry=ttu&g_ep=EgoyMDI0MDkwNC4wIKXMDSoASAFQAw%3D%3D)

+ in the wikipedia article (use a translator) in the description of the first phase this existing tunnel is mentioned. (https://sr.wikipedia.org/wiki/Pruga_Valjevo%E2%80%94Loznica)

(I am sorry for spelling mistakes...)

P.S. If this does not satisfy the need for justification, please comment & ask, I will be happy to answer & contribute what is in my power & try to find more refferences.
Thanks!
Theodore