Thanks, nightwalker - although the credit goes to Sidelights on Tranmere, which I recommend to anyone who's interested in the History
It would be interesting to know exactly at what point poor Corporal Greene was trying to cross the Pool. Was it near where the railway bridge was later built, or further east towards the Mersey?
It certainly looks like the 'illegal' rail route to Monk's Ferry is shown on the map and that sounds like an interesting story in itself.
However, the bridge at Waterloo Place / Blackpool Street was part of the original railway line and presumably would have spanned Tranmere Pool when it was still a tidal inlet. The bridge is still there today and you can see how the land was embanked either side to raise it across the gap.
As Derek said, this is the lowest geographical point in Birkenhead and Blackpool Street would have been the floor of Tranmere Pool.
Incidentally, when the gasworks was built nearby, tne waters of the Pool became very polluted. I wonder if that's why it was called Blackpool Street - not after Blackpool the place, but because the pollution made it a 'black pool.'?