The Split Track command provides several functions. It can be used to disconnect the joint between two sections of connected track, to divide a section of track into two pieces at a selected point or to place block gaps at an existing joint.

Until a split point is selected, the split point under the cursor will have a blue marker and if it is near a connected end a blue "X" will indicate that a split will occur.

For detail about;

Disconnecting the joint between two sections of track see Disconnect Joint.
Dividing a section of flex-track into two connected pieces or trimming a turnout see Divide Track.
Creating block gaps at an existing joint see Adding Block Gaps.