One of the most difficult problems in equip ping an electric railway system is the propor tioning of the total capacity of a power house to the number of cars run, and the subdivision of this total capacity into proper-sized units: In order to determine these points, it is necessary to be able to estimate the maximum loads that will have to be carried during the rush hours and the minimum loads during midday and mid night. The maximum load does not always de pend upon the total number of trains run, as on some lines more than half of these trains will be running light in one direction in order to carry the crowds back on their return or vice versa. In subdividing the total capacity of a power house, it is desirable to have a single unit which will carry the minimum light load with reasonable economy. The load on a power house operating in connection with an elevated railway system varies not only from a maxi mum during rush hours to a minimum during off hours, but also momentarily during the start ing and stopping of trains, these fluctuations being most violent when the least number of trains are in service. Under the worst condi tions, this rate will vary from 300 amperes to 4,000 in 15 seconds, or the reverse. It is neces sary, therefore, that all parts of the engine should be especially heavy, particularly the fly wheel, in order that sudden changes in the load should not interfere with the proper regulation of the engine speed.
The accompanying diagram, Fig. 1, shows a typical load curve at the power-house. Curve A is that of the trains in service. Curve B shows the additional amount of energy for heating the cars, with heaters only partly turned on during the rush hours; while curve C gives the total load with heaters turned on full dur ing the rush hours. The high peaks of the curve occur at the rush hours of travel.
The second diagram, Fig. 2, shows the vio lent fluctuations in the demand for electrical energy for operating the trains.
In the third-rail system the use of the ordi nary track rail for the conductor was largely a matter of convenience in the first case, as rails were the easiest form of steel to obtain in rea sonable lengths, and their shape was such as to lend themselves to the various requirements of attaching bonds, angle bars and insulating sup ports. The value of an ordinary commercial steel rail as an electric conductor, as compared with copper, is about 10 or 12 to 1, with of course the additional disadvantage against the rail of the necessity of the frequent bonding, the rails usually coming in 30-foot lengths. To offset this lower carrying capacity, however, compare rails at $17 per ton with copper at $360 per ton, and it can be seen that one can afford to put in the larger amount of steel re quired for a given electrical capacity and still have a good margin in favor of the rails. A commercial 80-pound track rail has a carrying capacity about equal to an 800,000 centimeter copper cable. In purchasing the contact rails for the extension of a western railway line, they were made of steel of a special chemical composition, having a higher electrical carrying capacity than the ordinary commercial steel rail. The com position was obtained after a series of experi ments conducted for the Manhattan Railway in New York, with a view to getting the best possible conductor with a composition of steel that could be successfully rolled into rails. The use of this composition results in a steel rail so soft as to be unfit for ordinary railway serv ice, but the conductivity is raised so that, com pared with copper, the ratio is about 8 to 1, as against 12 to 1 for ordinary commercial steel rails.