The following pictures illustrates the realisation of a circle through a pure fictional example. It is not based on a concrete project and no technical nor geographical details have been considered. It only shows the principle, how tracks could be made contiuous and cities could be circumnavigated.
In an real world implementation a circle would be constructed through a conjunction of high speed tracks that by pass the urban areas. However, the precondition of the constant speed can also be achieved on a more irregular line.
The extreme case is a straight line that would only be adjusted to drive around the city. With a given entrance time of a train and a constant speed the exact position of a train can be predicted on any continous line.
As 90 % of the tracks are overland, only a rather small new investement have to be done to bypass the cities.
The example connects several separated lines to a continuos line of tracks. They are illustrated with colors and names ( yellow, green, blue circle etc). As the picture demonstrates, circle can – or should be overlapping to optimize track usage and connecting wider areas.