From: Vancouver, BC
Eliminating that rail line to Sarajevo would probably stop the Montenegro Gambit stone cold, since it is all about deploying overwhelming force against a high value target in the first couple of moves of the game. If the CP player has to walk the attacking force over low supply, hilly terrain to surround Cetinje, they will probably focus on the main Serbian front instead, leaving the task of capturing Cetinje to later. If you eliminated that rail line, I would suggest it be paired with a modification to the French-Serbia convoy script that requires Serbia to control all the hexes of the rail line between Cetinje and say Pristina, so that the CP player would have other ways of stopping the convoy, by cutting the rail connection from Cetinje to Serbia. (I mentioned this thought in my post in response to yours on the Mod forum).
Looking at the map you have posted, it looks like the whole Dalmatian coast is "under-served" by the A-H rail system, not just the link from Sarajevo to Cetinje, and the same argument could be made about the capacity of the rail connections to the various ports along that coast-line. But I am inclined to leave that sleeping dog lie for now.