You don't hear much about new overhead railways being built, they are things that exist in New York, Chicago and in Liverpool's case the past, but new ones are surprisingly common. The Docklands Light Railway is largely overhead as are sections of the Newcastle Metro, European cities have or had some elevated railways, sections of Hamburg Hochbahn are elevated.
The very earliest overhead systems where simply brick viaducts, while the arch spaces could be used they took up a lot of space and often ran heavyweight trains. Later systems used lighter weight trains and lighter weight viaducts enabling the systems to be raised on steel or concrete columns. Leaving space beneath for roads. Some like the Liverpool Overhead were built above other railway lines, that ran along the street for freight transport between docks.The main advantage of the overhead railway is, that like an underground it leaves the surface free or largely free for other uses, such as roads, pedestrians or surface level tram lines. Compared with underground it is cheaper to build and easier to maintain, as well as requiring less safety systems. As they do not have to cooperate with other surface vehicles the can also have double the capacity of surface level trams.
Liverpool lost both its tram and overhead railway in the 1950s, things which, with hindsight seem like a mistake. There was even an attempt in the 2000s to create a new tram system as Merseytram, but that too failed, even after the purchase of the land and even the rails had started.
for an idea of station location on the LOR section see
ReplacementA large part of the original Liverpool Overhead Railway's business was messengers moving between the docks, the widespread of the telephone put an end to that. Changes in technology and the docks have meant that that need has not returned. However, the rise of the area as residential and the move of the retail area of Liverpool further towards the river may offer a source of passengers.
There are some other unused old rail resources in central Liverpool which could be brought into use to produce a more integrated system, especially if Tram-Train technology is thrown into the mix, a central network connected, via existing heavy rail lines to subsystems on the outskirts, can be created.
Original the LOR was an isolated system but it was finally connected to the main rail network with a link to the North Mersey Branch, which allowed connections to all the north line to Southport and the line from Aintree to Ormskirk, it would have provided access as far as Kirkby but the North Mersey Branch was not electrified that far. The change of use does not extend far beyond Sandhills Lane so there would be little case for extending the line back to Litherland. However, joining the Northern line near Sandhills could use the wider track section provided by the CLC route.
To the south, a connection to the Northern line at Brunswick could be made. Further links could be made by the Wapping Tunnel and the Waterloo Tunnel to Edge Hill, where a curve would create a triangular loop line. Extra stations could be provided along the length of the tunnel.
In order to use the tunnel, the line would have to drop down to ground level. The normal clearance required on motorways is 15.1 meters, if this is required for the new railway then this would put the rails some 17 meters above the ground. The steepest gradient on the Docklands Light Railway is 1 in 17 (5.88%), at that gradient, the ramps would need to be 119 meters long. This length easily fits into the available space where transitions are required.
Merseyrail already plans a connection from the northern line, south of Central station to the Wapping tunnel as part of it Edge Hill Link scheme, with at least one additional station at Crown Street. Part of their plan is to open up the area around Wavertree by rebuilding the LOR as Tram-Train and linking it to Edge Hill the link could be fully utilised and the Trams go beyond Edge Hill and at some point leaving the railway and mix with ordinary traffic as a street tram.
The tighter turns of light rail would allow, with some tunnel works a connection to the wapping tunnel heading west, though the floor would have to be lower to clear an intrusion into the tunnel.
The Waterloo tunnel passes within 400 metres of Lime Street underground on the loop line, a short branch from the Waterloo could provide a platform parallel to the current loop line platform and provide access to both Loop Line and Mainline passengers.
With a connection made to Lime Street then a short spur of a loop which passed down Princes' parade would allow the cruise liner terminal to be connected conveniently to the mainline station, with just a short walk.
It is difficult to estimate costs but I found Comparison of Capital Costs per Route-Kilometre in Urban Rail from a Danish university. Which contains this table
There are about 7km of existing tunnel, which as the tunnel is already built I have priced as surface, 8 of elevated and an extra 0.6 miles of new tunnel. All figures in millions.
|2000 → 2017 $||1.4016||2017 $ → £||0.81|
|Cost||Kilometre||2000 prices||2017 prices||2017 $ → £|
So roughly £275 to £685 million for the overhead rebuild or between £435 and £1045 million for the complete system.
|From Garcia Bridge Engineers|
which has quite a lot of similarities with how the original was built! Though instead of being counterbalanced the front is supported by mobile columns on their own railways.