Complex sentence: During the transition period, knuckle couplers on locomotives had a horizontal gap and a vertical hole in the knuckle itself to accommodate, respectively, a link and a pin, to enable it to couple to vehicles which were still equipped with the older Johnston couplers.
Split into simpler sentences: During the transition period, knuckle couplers on locomotives had a horizontal gap and a vertical hole in the knuckle itself to accommodate a link and a pin respectively. This enabled them to couple to vehicles which were still equipped with the older Johnston couplers.