तमन्ना जहाज के सफर की... क्यों करूं मैं, दिन भर की खुशियाँ लम्हों मे बर्बाद क्यों करूं मैं। - Amir

On a sustained long duration run, absolutely not. LP's are not allowed to over-speed under any circumstances on regular runs.
However, one thing to be understood is that the speed control on tap-changer locomotives is very coarse. It takes an incredible amount of control and precision from the LP to maintain conventional locos at a particular speed. (+/- 5 kmph is a typical variation for normal LP's).
On a flat route like HWH-DLI, maintaining the speed is relatively easy. But for example, if a...
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train is going at 109 kmph on a 110kmph section and hits a 1 in 200 down gradient, then the speed will go over 110kmph, because of the down-gradient and because the loco cannot respond instantaneously to the LP's commands.
It is this over-speeding that I am talking about. In such cases the LP usually gets the train back to normal speed within 30-40 seconds, but on routes with frequent gradients, they are given some prescribed time to get their locos back to the limit.
Note that such kind of brief over-speeding due to gradients is unavoidable for locos that do not have speed locking. On a route like SC-KZJ (120kmph route) with a lot of gradients, it is completely unavoidable for a train running close to it's speed limit to avoid crossing the limit. Some LP's are extra cautious and leave a 5 kmph margin from the MPS to avoid this, while some don't. However, such minor spikes are not taken seriously.