The New York Rapid-transit Subway

112 DISCUSSION ON NEW YORK SUBWAY. [Minutes of Mr. Copper- in the central part of London. Even if the physical difficulties thwaite. could be overcome, the cost of work of that description would be such as to render it quite prohibitive. Mr. Hudleston, who was experi- enced in the matter, had given some figures for the cost of tube railways, and had found that under the most favourable circum- stances the shallow subway might compare on even terms with a tube railway ; but in order to arrive at those figures Mr. Hudleston had omitted two points which might be referred to briefly. In the first place, Mr. Hudleston had assumed that the cost of the land was to be the same for a tube as a subway. In the London streets it was quite clear that a station of the type of Fig. 16, Plate 6, could not be built without land being acquired on each side of the roadway. The land on the Central London railway cost upwards of £100,000 per mile ; and if, instead of having a station on one side of the road, the station had been divided, part being placed on each side of the road, the cost, it was true, would not have been doubled, but it would certainly have been very much more. Land alone would probably have added £50,000 per mile to the cost of constructing shallow subways in London. Another point which Mr. Hudleston had disregarded in making his comparative estimate was the economy in working effected by the arrangement of gradients to suit the direction of the trains, a matter easily managed in a tube railway without extra expense, but impossible, save at great expense, in a surface subway. Although it was quite impossible to work by cut and cover in the present condition of London traffic, there was one way that had been tried in Paris and Boston whereby shallow subways could be built at a very fair rate of progress—what was called the “roof-shield” method of tunnelling. It was not as good as the ordinary shield method, because it required two operations, but it was comparatively simple, and it was possible to apply it without serious disturbance of large water-mains and gas- pipes. In Tremont Street, Boston, 24-inch, 12-inch, 10-inch, and 8-inch gas-mains and 18-incli water-mains had been tunnelled under for a distance of 550 yards, without difficulty. Two side headings were driven first, in which were built the side walls of an ordinary two-track tunnel. When the side walls had been driven, the semi- circular roof-shield was started on the top of the two walls, running on shoes. The earth was taken out of the crown, the concrete roof was put in, and the invert was built afterwards. Progress was made at the rate of about 9 or 10 feet per day—not at all bad progress for a double line of track; and the process had the great merit of not interfering in any way with the traffic overhead. When the same thing was tried in Paris an attempt was made to drive a tunnel as