The train tracks from St Pancras to the Channel Tunnel are known as HS1 (short for High Speed 1). They are the most recently built train lines in the UK and their design incorporates learning from previous lines as well as needs from improved technologies.
The moving with the technology needs relates to the speeds at which engines can pull trains along tracks nowadays. In the 1800’s, when many of the train lines were built in England, they were powered by steam engines which could only pull the trains so fast. Newer diesel and electric engines can power the trains at far higher speeds. The older tracks however have, in places, corners which are too tight for trains to travel along at such high speeds. HS1 on the other hand was designed to just not have such tight curves and hence the trains can travel along the tracks faster than most others in England.
The other unique aspect lies in the signalling system. It allows signals to be given to trains travelling in either direction along either track. So rather than all east-bound trains having to travel on one set of rails, as along the Great Western line, the flexibility exists for trains to travel eastbound on either track, say if there was an obstruction or engineering works or other unusual operating conditions.
The route itself passes along the Thames estuary and through the Kent countryside. I glimpsed marshland, ponds and white chalk which makes up the White Cliffs of Dover along the way.