Interesting. Per my understanding, locomotives were not allowed to exceed the speed listed on the certificates RDSO had issued for them. For diesel locomotives, I've seen that these limits are respected even on tracks that have a higher MPS and on trains where theoretically a higher speed of operation should be possible.
You can click here for a list of all speed certificates. Note that more recent ones supersede earlier certificates. Now I see that on Rajdhani routes locomotives are cleared for much higher speed but I don't see a WAP-4...
more... certificate on there. The latest WAP-4 certificate on mainline routes sets a limit of 120 kmph.
Does anyone know what the story here is?