This command is used to extend the length of a straight or curved section of track.

Click the Modify button ( ) to activate the Modify function.

With the mouse pointer hovering over the endpoint of the object to be modified, Right+Drag (Window Terms) to extend the length of the track segment.

The selected endpoint determines the position and angle of the first endpoint of the new track. The cursor is dragged to set the second endpoint. A curved or straight track will be created, depending on the position of the second endpoint.

If easements (Easements) are enabled then an easement curve will be generated (if required) to connect the old and new tracks.

For detail about;

Changing track length refer to Changing Track Length.
Creating a tangent to a curve refer to Creating a Tangent to a Curve
Changing track radius refer to Changing Track Radius