Dividing a section of track into two connected pieces;
It's not possible to create a track shorter than the Minimum Track Length (Section 2.10.7).
This command is useful for placing tunnel portals or elevation points (Section 2.2.4) at specific track locations.
Normally a turnout cannot be split however, if the Shift key is held when executing this command then the turnout can be trimmed. Trimming replaces the portion of track between the split point and the turnouts endpoint with flex-track. For turnouts with more than one set of points (Section B.2), such as a slip switch or scissors crossover, a trim point cannot be selected between the points of the turnout.
For detail about;