Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 311 This, of course, is true only if there be a station at each summit at which the train has to stop ; so noth- ing came of the proposal until Greathead applied this principle, as far as possible, on the original sections of the City and South London Railway. In all subsequent Tube lines, the Central Lon- don Railway particularly, this principle has been ob- served where practicable. A train on quitting a station at once strikes a downhill grade of 1 in 30, extending for such a dis- VARIOUS RELATIVE POSITIONS OF THE TWO RUNNING TUNNELS OF A TUBE RAILWAY. The disposition depends largely on the breadth of the thoroughfare overhead. Gradients on the Tubes. alone will be tance that, assuming the motors to be taking current until the train com- mences to descend the 1 in 30 grade, the speed at the bottom of the incline under gravity about 25 miles per hour. The train then proceeds along a level stretch of line, using very little, if any, current, and so continues to run at a high rate of speed to within a short distance of the next station, when an up-hill grade of 1 in 60 against the load acts as a brake to the train and rapidly checks it. The air-brakes have to do much less work than on an ordinary railway. This is not only theoretically the proper way for a train to run a section, but is also practically the best, as it enables a maximum average speed to be attained at a minimum cost of energy and braking. The two tunnels of a Tube railway, although generally placed side by side “ spectacle fashion, ’ are often placed one above the other. We may mention one notable instance. On the original section of the City and South London Railway, where the tunnels pass under Swan Lane, on the northern side of the river near London Bridge, the width of the lane being only just sufficient to admit of one tunnel passing along it without encroaching on the property on either side, the tunnels were placed one above the other. In such, a case, the precaution is generally taken of constructing the lower tunnel first. If the upper one were completed first, there would be considerable risk of its subsiding on to the lower while that Was being driven. The section of Tube railway which has the steepest gradients against the load of any in London is that of the Piccadilly Tube, between Earl’s Court and Baron’s Court, where the _ , . - Gradients. grade is 1 in 50 for 2,213 feet; but the Hampstead Tube has a longer grade of 1 in 60 for 4,333 feet between Hampstead and North End. The Hampstead Tube has also the deepest lift shafts of any line in London, those at Heath Street Station sinking 181 feet below the street. The greatest depth of the tunnel below the surface is near the White Stone Pond on the same railway, where the rails are 250 feet below the Heath.