On a deep-level railway, steam is, of course, out of the question. Even on the old 'Underground,' built close to the surface and furnished with frequent openings at the stations, and by means of ventilating shafts, the atmospheric conditions were abominable. The sulphurous fumes were indeed recommended for asthma and other complaints, but on a tube railway they would have been sufficient to cure every human ailment. Therefore the choice lay between electric traction and haulage by cables, compressed air, or some other innocuous system. Within these limits electricity was chosen on its merits. The first railway in Great Britain to undertake conversion was one in which both the physical and economic troubles were exceptionally serious. The Mersey Railway is little more than a tunnel under the river, and it is distinguished by heavy gradients and by the continuous necessity of pumping out the water which drains into it. With steam traction the difficulty of ventilating the tunnel was an added trouble. Owing to these various causes the working expenses were abnormally heavy, and led ultimately to a receivership. Electric traction was adopted as the only possible cure. The pumping and ventilation arrangements were both reorganised for electric power; and the trains were equipped with electric traction on the 'multiple-unit' system, an arrangement--to be described in the next chapter--which is well suited to the economical handling of steep gradients. The practical result was a great increase in traffic, with a marked decrease in the proportion of expenses to receipts. No other British railways, happily, were in so desperate a condition as the Mersey line, but all of them were, at the end of last century, feeling the effect of certain disquieting tendencies. These tendencies were most marked in connection with suburban and short-distance inter-urban traffic, which is quite distinct in character from the main-line traffic. We talk glibly enough of railway traffic as if it were a unity, but it is clear that very different considerations govern the traffic on a main line between, say, London and Glasgow, and those which control the traffic on London suburban routes or on a railway connecting the adjacent towns of the Potteries. Some railways have to deal with all three classes at the same time and occasionally on the same lines of rails. Electric traction has, so far, made itself felt only where the suburban or similar inter-urban traffic has been separable from the main line traffic.