Excerpt from The City and South London Railway, Vol. 1: With Some Remarks Upon Subaqueous Tunnelling, by Shield and Compressed Air

There are three pairs of pumping engines, so arranged that each can be worked independently of, or jointly with, either of or both the others, and controlled automatically by the accumulator in the usual way.

Permanent Way - It was thought to be very desirable to avoid the introduction of ballast on this line with a view to the comfort Of travellers, having regard to the strong air currents set up by the trains, and to the durability of rolling stock, especially Of the electric locomotives, and of the way itself. The rails, weighing 60 lbs. Per yard, are placed upon cross sleepers resting directly upon the cast-iron segments of the tunnel, Figs. 14 and 15, Plate 3. The rails, it will be observed, are single-headed and are set some distance from the ends of the sleepers. This permanent way, though subject to the disadvantage of increasing the resonance in the tunnel, gives remarkably good results as regards repairs and maintenance.

Ventilation - One Object in placing the up and down-lines in sepa rate tunnels was to secure ventilation by the action of the trains. Where trains pass to and fro in the same tunnel their action in renewing the air in the tunnel is comparatively slight, unless frequent passages of short length between the outer air and the tunnel be provided. Any considerable transporting action of the train extends only for a short distance in front of and behind the train, hence the great importance of what have been called the blow-holes on the Metropolitan Railways. Where, on the other hand, separate tunnels are provided without an escape for the air, except through the stations and their passages, the draft or blast of air on the approach and departure of a train having a cross-section large in proportion to that of the tunnel, becomes inconvenient and a source of discomfort, while the resistance offered by the air to the movement of the train is considerable. The first objection, but not the last, can be obviated by providing openings to the air of sufficient area at each end of the station platforms. Both can be avoided by placing connecting passages between the tunnels, capable of being enlarged or reduced, for regulating the through draft. This expedient was adopted on the City and South London Railway, and with quite satisfactory results, because in the absence of steam locomotives it is not necessary to renew a large quantity of air on the passage of each train. Suflicient provision must in such cases be made at each station for the successive outflow and inflow of air.

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