You are right about controlling and Signalling. All I am saying is that since the NGP-ET line is a trunk line, it is very likely that they won't stop with just rail upgrades, but also go ahead with the remaining upgrades too. Also, the NGP-ET line has been a trunk line for a very long time. The railway operations on the route have long since evolved to handle high speed passenger traffic. So many aspects of signalling and controlling are already well adapted for handling high speed passenger trains. The track bed, bridges and other civil engineering works are already capable of handling higher speeds. Compared to many other 60Kg routes, this one already has a lot of infra already setup or well in progress.
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The rails are not different, but the purpose is. Rail maintenance schedules are usually determined by the amount of traffic on the route. Heavier rails allow running heavier axle load trains at higher speeds than lighter rails. Now, it is up to the railway to determine how they want to use that additional capacity. The VSG-GTL route for example, utilizes most of that additional capacity for running heavier freights. In addition, since it is a diesel operated route, high speed runs for freights are difficult to achieve, so they chose to use the heavier rails for heavier freights.
Note that many of the other routes that have 60Kg rails do not necessarily have the other required infrastructure for allowing high speed runs. Not just signalling and controlling, this also has a lot of civil and mechanical engineering works and even OHE related upgrades. 60KG upgrade can be done without changing these other parameters. These are routes where the heavier rails are used just for freight loading.
On the other hand, the NGP-ET line, or for that matter, long stretches of the 6 trunk routes are already laid down with many of the basic requirements for high speed runs with many of the civil engineering works already complete. The necessary upgrades to be done to these routes is much lesser than the non-trunk routes.

Well, you are mostly right. But heavier rails are necessary for higher speeds as well. For example, now IR operates a lot of CC+8+2T freights with 22.82 ton axle loads. On 52Kg tracks, these heavy freights cannot run beyond 75kmph. An upgrade to 60kg allows them to be run at 100kmph. And similarly 60Kg rails cannot handle 30 ton freights (to be run on the DFC's) at 75kmph, so heavier or stronger rails are required for the DFC to allow both heavy loading and higher speeds.
The curves, PSR's, TSR's etc are equally important, but then this is a trunk route, so unlike branch lines or freight-exclusive routes, such issues are handled regularly and the trunk routes...
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see regular upgrades for improving their traffic throughput. Yes, these too have to be taken care of before the 150kmph upgrade for the route, but it is going to be a much easier task (and already in varying degrees of progress) for a trunk route.