Mr. J. M. Fraser, in a paper which he read at the Institution of Civil Engineers, on the 24th of March, 1863, considered, that “it would not be inaccurate to assume 80 miles as representing the length of tunnels now daily traversed by railway trains in the United Kingdom.” At that period there were 11,547 miles of railway in operation. As there were 13,882 miles opened for traffic on the 31st of December, 1866 (of which about one-third is single line), we may consider that w e have about 90 miles of line “in tunnel” at the present date, and in these are included the Metropolitan (Underground) Railway and the tunnels[127] on the New Midland Line between Bedford and London, opened for goods traffic in August 1867. We knew as a fact that, during the early construction of railways in England, our engineers resorted to tunnels to avoid gradients and curves (especially the former) that would almost be considered favourable at the present day. As experience increased and the power of the locomotive was developed, so did the amount of tunnel work diminish. Hence it is that if the three estimates just stated be correct, the proportion of tunnel to railway on the 1st of January, 1856, was one mile to every 115; on 1st of January, 1863, one to every 144 miles; on 1st of January, 1867, one to every 154. Had Ireland been excluded from the reckoning,—as she might well be, seeing that there are only three there of the aggregate length of 2,980 yards (exactly 100 yards less than a mile and three-quarters) on the 1,948 miles of railway that are now open for traffic in that part of the United Kingdom,—the proportion of tunnel to railway in England, Scotland, and Wales would have been one mile of the former for every 132½ of the latter. The number of tunnels in Ireland is, however, it is alleged, about to be added to, as, since a very serious accident at “Bray Head,” on the Dublin, Wicklow and Wexford Railway, a feeling of uneasiness prevails in the public mind as regards the safety of the line at this point. It is, therefore, contemplated to run four tunnels through the mountain at its side nearest to the sea, at an estimated cost of about £23,000. Bray Head, it may be mentioned, forms the southern boundary of the Bay of Dublin, the Hill of Howth being its boundary to the north.
The longest railway tunnel in England[128] is the Woodhead or Summit Tunnel of the Manchester, Sheffield and Lincolnshire Railway. Its length is 5,296 yards, or 3 miles and 16 yards, with a gradient of 1 in 200, or 26⅖ feet to the mile, the rise being in the direction from Dunford to Woodhead. The ordinary passenger trains require ten minutes from Dunford to Woodhead, but trains travelling from Woodhead to Dunford take a minute less. Therefore, with the gradient favourable, the speed is only at the rate of 20 miles an hour; against the gradient, 18. It is a single line tunnel.
The Stanedge Tunnel on the Huddersfield Branch of the London and North-Western Railway is exactly 3 miles long, and it is also a single line tunnel; the gradient is nearly a level. The time for passenger trains through it is six minutes, or at the rate of 30 miles an hour; the time for goods trains is nine minutes, or at the rate of 20 miles an hour. The deepest portion of the tunnel is 600 feet below the upper surface of the mountain.
On the 3rd September, 1867, we came through the Shepherd’s Well Tunnel of the London, Chatham and Dover Railway, 2,376 yards long, against the gradient, which is 1 in 100, or 53 feet in the mile for two-thirds of its length, and the remainder nearly level, in two minutes fifteen seconds; on the same day we came through the tunnel in the opposite direction, when the time occupied was one minute forty-two seconds: on the other hand, Mr. Allport, the General Manager of the Midland Railway, to whom we gladly acknowledge ourselves indebted for much valuable information most obligingly afforded, says that the passenger trains are timed to run in each direction through the Dove’s Hole Tunnel 2,420 feet long at the same rate of speed, and actually do run at that speed—nearly 46 miles an hour,—yet the gradient is 1 in 90, or 58⅔ feet per mile. The same for goods trains, their speed is 23 miles an hour in each direction, and Mr. Allport assures us they maintain it in both directions.
The following are the lengths of some of the principal railway tunnels in Great Britain and of the two longest in Ireland:—
(A) Followed by an open cutting of 100 yards; then a tunnel of 440 yards. The gradient in these tunnels is 1 in 70. For full details respecting them, see Paper read by Mr. Charles Nixon at the Institution of Civil Engineers. Proceedings, volume for 1842.
Some of our tunnels have been extremely costly, the Kilsby, for instance, £125 per yard forward, total £302,000. The face of the Primrose Hill Tunnel at its London end cost £7,000, thereby adding to its total cost at the rate of £5. 15s. a yard. Up to 1857, the average cost of all tunnels was £102 per yard, or for 70 miles £12,320,000. Tunnels have certainly been constructed more cheaply since then.[129] Mr. J. G. Fraser assumes them to have cost at the average rate of £45 per lineal yard, but this must be considerably below the mark, notwithstanding that many of the more recently constructed tunnels have only been for “single line.” The dimensions of these tunnels on the 4 feet 8½-inch gauge, are usually as follows:—12 feet wide at rail level, 17 feet from rail level to soffit, whilst for a double line of the same gauge tunnels are usually about 25 feet wide at rail level, and about 22 feet from rail level to soffit. On the exceptional Great Western gauge, they would be respectively about a fourth higher and a third wider.
We must only touch the Thames Tunnel at a tangent. Its length is 1,200 feet with two arches each 13 feet 9 inches at the springing of the arch; 16 feet 4 inches high from the invert. It cost £1,000 a yard, or for 400 yards £400,000, two and a-half times as much per yard forward as the most expensive estimate for the Mont Cenis Tunnel, more than three times as much as its cost per yard forward up to the present time. In addition, the approaches of the Thames Tunnel (for foot passengers only) cost £54,714, total £454,714. It is now about to be utilised as a railway tunnel, having been purchased by the East London Railway Company for the purpose of bringing together the lines at the north-eastern and south-eastern ends of London.