[Illustration: 1]
[Illustration: 2]
[Illustration: 3]
[Illustration: 4]
[Illustration: 5]
[Illustration: 6]
Only six reversals are necessary. The white train (from A to D) is
divided into three sections, engine and 7 wagons, 8 wagons, and 1 wagon.
The black train (D to A) never uncouples anything throughout. Fig. 1 is
original position with 8 and 1 uncoupled. The black train proceeds to
position in Fig. 2 (no reversal). The engine and 7 proceed towards D,
and black train backs, leaves 8 on loop, and takes up position in Fig. 3
(first reversal). Black train goes to position in Fig. 4 to fetch single
wagon (second reversal). Black train pushes 8 off loop and leaves single
wagon there, proceeding on its journey, as in Fig. 5 (third and fourth
reversals). White train now backs on to loop to pick up single car and
goes right away to D (fifth and sixth reversals).