This is the Carrera Evo/Digital 20585 10 Meter Power Supply Extension Cord

Additional Supply, 10 m
As tracks of an entire length of 15m/49.20 ft. and more might experience a power loss it is recommendable to use an additional power supply. Additional power supply minimizes the transition resistors while the slot cars are being supplied with sufficient power. The additional power supply cables required are available in a length of 5 m/16.40 ft. (item no. 20584) and 10 m/32.80 ft. (item no. 20585) via the specialised trade. Starting from the connecting track an additional supply should be implemented every 5-10 m (16.40 - 32.80 ft.). It is not possible to fit a second connecting track.