1889 To begin work, an Act of Parliament was necessary (the Westminster Parliament, no devolution in those days). The legislation was opposed by the Clyde Navigation Trust, the partnership of the council in Glasgow, shipowners and the Glasgow Chamber of Commerce with responsibility for developing the river for business. The stated reason was that a tunnel could prevent future deepening of the river, and it was agreed any tunnel had to be at least 15 feet below the river-bed. I do wonder if the Trust was opposed because this wasn’t their idea and was to be run by ‘outsiders’.
The Glasgow Harbour Tunnel Act 1889 received, Royal Assent on 12th August
Glasgow Harbour Tunnel Company formed, based at 163 West George Street
Issues shares at £10 each, initially to value of £100k, then increased to £135k, fully subscribed
Business plan: assumed revenue of £10,600 per year made up of two elements:
2000 vehicles per day to use tunnel at a charge of three pence, plus
Passenger ferry made £5590, assume half the revenue switches to the tunnel
Working expenses and management cost £2800 per year
After allowing for interest on mortgage, there will be enough to pay a dividend of 4.5%
Assumptions justified on the basis 10,000 vehicles use Glasgow Bridge every day and 11,000 people use the Stobcross ferry
1890 work begins on the south end in January.
By April, 25 feet (7.6 metres, about 30% of the planned total) had been excavated, with work proceeding day and night. Being dug by hand, mainly through sand, with material taken out of the shaft by crane. Work at north end to begin in May (actually June).
Costs to date: £6k for costs associated with Parliamentary legislation, £9k to purchase land on either bank for entrance/exit
By October, south shaft ready to begin tunnelling under the river. This shaft had been lined with iron to 55 feet and was being lined within that with brick. North shaft down 30 feet.
Company chairman quoted as saying, “Everything that had happened since they started the company for the formation of this tunnel had gone to satisfy them that it would meet a felt need, and that it ought to be, and would be, a most important means of communication. In recent the recent Parliamentary action of the Clyde Trustees in regard to the Cessnock Dock and the form which that undertaking had finally taken , made this harbour tunnel a most important link between the docks on the north and on the south of the river. It was therefore with full hopes of success, both as to construction and as to the traffic which would be obtained when the tunnel was constructed, that he moved the adoption of the report and the accounts.”
Decision to take out a mortgage of £45,000 to cover costs.
1891 April: shafts 74% complete, tunnels 8% complete
Built using compressed air to force out water, 20 men working per tunnel. Vehicle tunnels dug first.
1892 May: vehicle tunnels nearing completion. Work on passenger tunnel set to start.
Engineers preparing tender for the lift machinery
October: vehicle tunnels (still) nearing completion. Passenger tunnel “in an advanced state”.
Contract for lift awarded to an American company
1893 April: passenger tunnel about halfway across the river, expected to complete in October.
Work had started on the iron framework required for the lift machinery.
October: vehicle tunnels complete, passenger tunnel within a few yards of completion. One shipment of lift machinery had been sent, the second and final shipment left New York on 12th October. Handover from lift company expected on 1st September 1894 “in thorough working order”
1894 January: total cost of iron plates for the shaft and tunnels put at £24,748 17s 2d
October: framework and machinery for lifts in place. Accumulator houses had been delayed by difficulty obtaining bricks during a miners’ strike, but now finished. Hoping to open early in 1895. Just setting up electric lighting in the tunnels.
1895 July 4th: At the dinner to mark the opening the Chairman said five years to complete was “far too long”.
Great confidence was expressed that there was a demand for another way to cross the river. One of the directors, J Parker Smith MP, said “facilities make traffic” (i.e. if you build a road or tunnel, people will come forth to use it).
One of the directors opined, “In the light of what they had done, the engineering enterprise exhibited in the Tower Bridge [in London, opened the previous year] had been shown to be absurd.”
The assembled directors drank a toast to their two guests from the Clyde Navigation Trust. The Chairman said it might be thought that the company and the Trust were in opposition but it was not so. He had had very civil dealings with them “more especially with Mr Deas, who on all occasions had met him as one gentleman ought to meet another”. In response James Deas said the tunnel had been constructed in an admirable manner, creditable to the designers and contractors.
In retrospect, it was significant that the tunnel company had dealt with James Deas and that he replied; he was the chief engineer of the Trust. The man who sat silent alongside him was T.R. McKenzie, Secretary and General Manager who was planning the Trust’s response; he would be their nemesis.
1895 July – Tunnels open for business, to pedestrians on the 8th and vehicles on the 15th. Fares were one halfpenny for a single journey or a book of 24 tickets for sixpence. Vehicles cost two pence. (cutting from Glasgow Evening Post 6th July 1895):

Total cost of building the tunnels was £238,000 as at September 1895
The land on the south of the river had cost £8,700 and £66,335 for the land on the north side. Of the latter only £10,000 was for land used by the tunnel buildings, the remainder being surrounding areas and warehouses for rental income.
To give this some context, the Glasgow Subway (a loop of 15 stations) was completed in 1896 for £1.5m and Tower Bridge in London was finished in 1894 at a cost of £1.1m.
On the eve of the opening, Clyde Navigation Trust reduced the prices for ferry trips by one-half and the tunnel was forced to lower its prices as well.
1895 October – Initial traffic was 1500 vehicles per week, rising to 3000 per week. Pedestrians 20,000 per week.
Newspapers were reporting despondency on the part of the Glasgow Harbour Tunnel Company (Manchester Evening News of 23rd October 1895):

“The directors had under consideration various plans for conveying passengers by mechanical means from one side to the other in the passenger tunnel.” This seems to be the basis for the story in the Railway News of 26th October 1895:

1896 Clyde Navigation Trust reduces fares for vehicles to one penny
Some property on north bank rented out for £500 per annum but the intention was to sell it
1897 January – company seeks to issues shares to raise a further £135,000 to cover loans taken out during construction work
April: Chairman tells the shareholders’ meeting there is no prospect of ever making a profit faced with the Clyde Navigation Trust determined to drive them out of business and backed by the ratepayer’s pocket
Chairman William Weir (link)
October: fee for a loaded cart raised to three pennies with unloaded two pennies. Ferries reduce price to one penny per vehicle amid concern that they would become free
£9000 spent on construction over the past six months including £1000 for additional pumping machinery
1898 engineers’ strike further affects revenue
1900 on 21st March, after nearly five years in operation, the Dundee Evening Post reported:

£50 spent on a weighbridge for carts
1901 director William Laird dies (14th August)
1907 January, meeting with CNT to ask for their fares to be returned to pre-tunnel levels. Declined to do so.
March half-yearly meeting no alternative but to wind the company up.
Looking back to the 1889 projections, the assumed revenue was £10,600 per year; the reality was probably less than half that amount.
It had been assumed 2000 vehicles per day would use the tunnels; the reality was about one-third of that amount.
It had been assumed passenger revenue would be about £2,795 per six-months; the reality was that passenger and vehicle revenue combined did not reach that level for any period where data are available.
Costs were estimated to be £2800 per six-month period; the reality was nearer to £3800.
As a result, the tunnel covered its running costs in 12 of the 16 six-month periods between 1895 and 1907 for which data are available – but as the company lamented the interest payment on their mortgage was between £650 and £750 per six months so losses just kept on accumulating.
May 1st, tunnels closed
While they had been unprofitable, they had been used by thousands of people and they were now left with no choice but to use the ferries. Faced with queues, the Clyde Navigation Trust opted for bigger ferries.
1908 July New ferry comes into service:

Cutting from the Port Glasgow Express of 24th July 1908.
1911 October, directors write to Glasgow Town Council saying they are prepared to reopen the tunnels under certain conditions
On 31st October the Daily Record reported:

1912 September: reported at company meeting that discussions still underway, trying to agree a price
1913 Negotiations took 18 months, but the Glasgow Harbour Trust Company had such a weak hand, they eventually capitulated (Financial News 20th February 1913):

The ceremony to reopen the tunnels took place on 1st July 1913 (reported in The Scotsman of the following day):

So the tunnel “would be of great use”, having effectively driven it out of business and kept it closed for 6 years? This does seem to support the idea that the Corporation’s opposition to the tunnel in the first place was a ‘turf war’ issue to exclude private investors.
The tunnels actually reopened on 10th July at a one-off cost of £1262, Passage was now free.
September(?): the chairman of the company, William Weir, dies
1914 March: Finnieston Ferry revenue falls to £4171 (July to February), down £2341 on the corresponding period in the previous year. Attributed to free tunnel – despite this the new arrangement for the tunnel was judged a success.
November: formalised arrangement, leased for ten years for a payment of £3000 per year
1915 The deal is extended for ten years with the Corporation leasing the tunnels for £3000 per year.

Cutting from Airdrie and Coatbridge Advertiser 27th March 1915
1916 January: director Andrew Kirkwood McColl died
1921 April: tunnels temporarily closed by mining strike threatening coal stocks
1924 annual cost of maintenance (to the Corporation) put at £20k per year – unclear how this had increased from around £5k when run by the tunnel company …
Consideration given to scrapping the tunnel and using the land at each end as the basis for a bridge. Expected lift machinery would raise £5 to 10,000 if sold.
1925 May 21st: the tunnel company goes into liquidation
1926 April 30th Corporation buys the tunnel land, works and rights.

Cutting from The Scotsman 1st May 1926
Intention to build a bridge of reinforced concrete 70 feet wide, carriageway 46 feet wide allowing two tramways. Central span of 200 feet width, with 70 feet between river piers and quay. Headroom of 76 feet at High Water Level of an ordinary spring tide – allows all current shipping using upriver quays to continue to do so
1927 April: final shareholders meeting. Paid £5 4s or £2 6s per £10 share (preference share vs ordinary share) – if I have understood this correctly, the investors lost between 50% and 75% of their initial investment. In addition, they could have invested it elsewhere to make a profit so this was an additional loss.
The tunnels after the Tunnel Company: managed decline
1930s at some point the pedestrian tunnel was closed and a water main built in part of the passageway.
1939 Sept 26: tunnel reopening (unclear when it closed)
1940 Feb 3: closed for traffic “until further notice”
1943 May 4th: tunnel closed for repair work “for an indefinite period”
October 11th passenger tunnel reopens “night and day except for 9am to 4pm on Sundays”
1946 survey finds 7000 people a day use the tunnel, rising to 20,000 on days of football at Ibrox or Hampden
1980 final closure of the foot tunnel. £1m cost to upgrade, used by about 100 people a day. Original proposal was to demolish the rotunda and fill in the shafts.
1986 term North Rotunda used in story about restoration as a restaurant
Vehicle tunnels filled in
Making the tunnels more useful: monorail, bridge or cars?
As we have seen, as early as three months after it opened, the directors considered whether some form of railway could be installed.
In 1919 the Corporation listened to what seems a fairly mad scheme to install a monorail in the tunnels (The Scotsman 20th August 1919):

As the tunnel was roughly 200 metres long, taking about three minutes to walk, it was unclear how much less time would be taken by climbing on a train, waiting for it to be full, then travelling through the tunnel and getting off.
In 1926, the idea of a bridge was considered again:

Cutting from the Oban Times 3rd July 1926
1931 Council receives a report it commissioned which says a swing bridge would be the best option. To date it had been assumed it would be a fixed bridge and had to be such a height that surrounding property was purchased to be demolished for the approach roads.
1935 report that bridge is planned, but cannot start before 1936, cost £400k so far in purchases, will cost a further £800k to build
1938 consideration of making each lift shaft into a circular road for cars to use the tunnels

Cutting from The Scotsman of 1st November 1938.
In 1956 the idea of a spiral access ramp for motor vehicles was still being discussed (from The Scotsman of 14th August):

Note the final paragraph – the Whiteinch-Linthouse tunnel would, of course, become the Clyde Tunnel that is still in use today.