Signal; UDX pattern 4 aspect combined tunnel signal and repeater, Westinghouse Brake & Signal Co. Ltd, 1929

In tube tunnels, sharp turns and gradients can make it difficult for drivers to see very far ahead. So signals sometimes have a repeater placed in advance, so the driver knows what to expect. If the block sections are short, the repeater may be placed as far back as the previous signal. In such cases a signal of this type was used. The top two lights - green and red - refer to the block section immediately ahead. The bottom two - green and yellow - repeat the next signal.