the berlin to baghdad railway wasnt exactly empire building except in the purely economic sense. Germany was not trying to turn the ottoman Empire into a colony, mpre liike a client state. As for the Balkans, it respected its ally's territorial sphere of influence.
Well of course turning the Ottoman empire into Germany's client state would not be looked on favorably by the British (or the other triple entente parties). As the colonial powers showed, economic empire building was often followed by a more aggressive form of de facto
or even de jure
occupation (hence the reference to Rhodes and his railroad). In either case it would have been perceived as a threat to British (and French) interests in the middle east. The Russians would not be happy either or both because of their cultural and religious solidarity with the Balkans and/or their concerns about the possible effect it would have on the control of the Bosphorus. The Ottoman Empire at that point was the "sick man of Europe" and did not pose a threat to Russia in this regard but direct or indirect German control of the straits could pose a grave threat. Plus the Ottomans were still in control of a large swath of the middle east and impinged on Britain's position in Egypt and Mesopotamia.