The river is 90 foot deep at low tide at the point the tunnel runs under, with 32 foot tides. The top of the tunnel is 11 foot below the river bed. The Mersey Railway was the world's second underground network. It was the first deep level underground bored in sold rock. Even the stations are bored out of solid rock - the first in the world. London being cut & cover tunnels and stations at the time. The tunnel was to extend to Woodchurch in outer Birkenhead and Birkenhead Central station was designed to accommodate the tunnel - but it never happened. That is why the train do a sharp left out of Birkenhead Central station and onto Green Lane, as straight ahead was to be the Woodchurch tunnel. That tunnel would have really made Merseyrail on the Wirral into a true district metro with most of the town covered.
The gradient is 1 in 28 on the tunnel stretch between Hamilton Square and James Street stations beneath the Mersey. The steepest incline of ALL on the entire British rail network.
Just before James St, short dead-end branch tunnels were cut into the Mersey rail tunnel to extend to Huskinson Dock. Again this was not done. One of the branches was used for the Wirral Loop tunnel in the 1970s. The remaining branch tunnel is still there and can be seen on the right from the train when approaching James St station from the Wirral. It is on one of the diagrams I posted.
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James St station at ground level rises from Mann Island and the high-tide mark is around 8 foot below the quays at the river. So, that is 140 foot below the river quay mark at the Pier Head. The tunnel is probably still the deepest part of any urban railway in the world - that needs checking. The station, along with Hamilton Square is very deep. So deep only escalators could manage the depth. The river tunnel is rising all the time and hit just below ground level I would assume around Church St - missing the old castle foundations. I'm sure they would have cut right through them if they had to.
Was the castle banked up to give a commanding height? That was quite common. Or was the ground naturally raised and skimmed later to level it?
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