A gathering line (also called a ladder track) is a track where a number of parallel tracks gather or merge. Alternatively, a number of parallel tracks also branch off from a gathering line. A gathering line is defined by the turnout angles and the angle of inclination of the ladder track to the parallel tracks (Fig. 15.14).
Gathering line at crossing angle
When the angle of inclination of the gathering line is the same as that of the turnout, it is said to be laid at the angle of crossing. In this situation, there is some gap between the back leg of the crossing of the turnout and the stock joint of the next turnout and a closure rail has to be used. The angle of the ladder track being equal to the angle of crossing, the two tracks intersect at the theoretical nose of crossing and no curve is introduced at the turnout crossing to connect the parallel tracks.
Gathering line at limiting angle
In this case the angle of the gathering line is greater than the crossing angle and a curve follows the back leg of the crossing. The back leg of the crossing is followed by the stock joint of the next turnout and no space is wasted. The limiting angle of the gathering line is given by following formula:
. Space between two adjacent parallel tracks
Sine of limiting angle =-
Overall length of turnout
Gathering lines can also be laid at 2a or 3a, i.e., at twice or thrice the crossing angle. Such gathering lines are generally found in marshalling yards and are known as balloon layouts. This layout of a marshalling yard based on the Herringbone grid is used when the various sidings of the marshalling yard are almost of equal length. This is not a very popular design (Fig. 15.15).