Ok, after playing around a bit, the problem appears to be two-fold.
The first issue is that when track built for 120km/h gets upgraded to 300km/h, there is no way to tell what the actual max speeds on this track are going to be. Furthermore, it seems that sometimes even though the track appears to be visibly upgraded, there are invisible sections that remain speed restricted. This is conjecture on my part, but in absence of any additional evidence, I believe it to be a fact.
The second issue is with train braking. Trains that travel at a velocity greater than the Vmax of a given section of track will slow down to 80km/h (or thereabouts) in order to negotiate it before accelerating again. As long as your track Vmax is above your train’s traveling velocity, this will not happen. So, even if you design a stretch of track that has, say, a Vmax of 230km/h, but the train approaches is at a higher speed, say, for example, 245km/h, it will not slow down to 230km/h, but will instead brake and continue decelerating to around 80km/h, even after long having cleared that section of the track.
The only workaround to these issues is to rebuild the entire track from scratch, making sure there are no stretches of track or curves with a Vmax lower than that of the fastest train you plan on using on that particular section of the track.
Anyway, I’m done with TF seeing how they’re done with patches and these issues have neither been acknowledged nor fixed in the final release. Sadly, I will not be buying further titles from this developer, as it is glaringly obvious they are perfectly happy with leaving game-breaking issues unresolved.