There was such a subway run by Hammersmith Council which as well as
linking the D&P and Met stations also housed two public conveniences.
Both straircase entrances were on the pavement and thus did not provide
a rain-free route between the two sites. I think it got a bit
disreputable in the evenings and was prone to flooding on occasions. I
guess it was felt safer to replace it by pedestrian crossings at street
level after the Hammersmith shopping centre scheme was completed.