b/w glass neg, crossover tunnels at the Kennington loop, by Topical Press, 1926

One in series U3938-U3941 taken of crossover tunnels at the Kennington loop (Northern line) where the two separate branches of the same line converge, described in log as tube within a tube. Inside a larger size crossover tunnel of 27feet 6ins in width, a narrower section of tunnel is positioned by an array of support beams. The bottom part of the mouth to tunnel from other converging branch is visible at end of the larger outer tunnel.