A <b>no-stopping zone</b> is a track section where trains are allowed to roll through but not completely stop and wait. A train can only enter a no-stopping zone if it can clear it.
Sometimes, no-stopping zones are not needed and removing them can improve train traffic flow. For example, when two tracks join and no other forks follow for a while, trains can safely wait on the merge.
While it is possible to design a train network where most intersections don't need no-stopping zones, there are cases where using them is vital to avoid trains getting stuck or waiting needlessly. For example, when overlapping tracks have different directions, like passing sidings on bi-directional tracks, no-stopping zones are needed for proper function.
There are also cases where adding an extra no-stopping zone can benefit traffic flow. For example, when you build a round-about, a small train may be able to stop and wait in the middle of it (1) while blocking traffic that could otherwise pass through (2).