ever, will reveal the fact that it possesses serious defects. The traiier is not automobile and requires the services of men and horses to attach it to the cars and to move it between the car sheds and the motor cars. The opening between the motor and the trail car increases the danger of accident both to passengers and conductors. The number of entrances and exits is increased and this augments the work of the conductor in keeping track of his fares and increases the danger of his missing some of them. If an extra conductor is placed on the trailer to collect fares, a great portion of the gain due to the trailer system is lost. The use of trailers throws an additional strain on the motors, as a heavy weight is added to the train which is not available for traction. The trouble is intensified as the load on the trailer increases; it makes the train more unwieldy in handling, and is largely responsible for the difficulty in starting and stopping quickly and in making schedule time.

To obviate the necessity of using trailers, a large car equal in seating capacity to the combined capacity of motor and trail car may be operated. This system, however, introduces the disadvantage of the operation at all times of the day of a seating capacity needed only during a few hours of the day. It also increases the size and weight of the cars and the average power required to operate them.

In order to compare the relative economies of single and double truck cars, their advantages and disadvantages will be discussed with reference to the following points:

6. The seating arrangements and convenience of exit and en

trance.

7. The preferences of passengers.

This relation will as far as possible be reduced to a money basis, and in order to do so a comparison will be instituted between a road which operates double truck cars and one operating single truck cars using trailers twenty-six per cent of the time to take care of the heavy night and morning travel.

The bodies of the double truck cars are twenty-six feet in length and contain eighteen cross-seats, seating thirty-six passengers. ne trucks are of the maximum traction type. The empty car weighs 23,500 pounds; and the motors are G. E. 800. An illustration of this car is shown in Fig. 4.

The single truck car has a body twenty feet in length; it is equipped with longitudinal seats, and the entire car weighs 16,000

pounds. It will seat twenty-eight passengers. During the period of heavy travel, which amounts to twenty-six per cent of the time, trailers are attached to these motors cars. Open trailers are operated in the summer months and closed trailers during the winter. The aw erage seating capacity of the unit, estimating the trailer as a part of the car during the time that it is operated, is thirty-five seats, which approximates the seating capacity of the double truck car very closely. The motors in use on the single truck cars are W. P. 50.

The number of cars operated is obtained by dividing the daily car mileage by 115. It is necessary to do this because the average mileage per car on the two roads is different. The comparison is thus between seventy single truck cars and forty-seven double truck cars, each car making 115 miles per day. The road operating the double truck cars has the greater density of traffic. The single truck car with the trailer attached is shown in Fig. 5.

WEAR AND TEAR OF TRUCKS.

It is evident that since a car equipped with double trucks has eight wheels and one equipped with single trucks has four, the former will be the more expensive to supply with wheels. Table I shows the comparative replacement and cost of wheels and brake shoes on the two roads for one year. Table II shows the comparative cost of maintenance and repairs of trucks and motors for two years. It will be noted that while the cost per car is much greater on the road using double trucks, the cost per passenger is almost the same. It would not be fair to assume from these figures that the cost of maintenance of trucks and motors per passenger is always the same, no matter what sort of truck is used, because these figures result largely from the fact that in this particular case the double truck car carries the greater number of passengers, but an inspection of the table will emphasize the advantage of the large unit as compared with the small one. It will be seen by a reference to table I that the greater part of the increased expense of maintaining the double trucks is due to the renewals of wheels and brake shoes.

In examining the relative strength of the various parts of trucks, the side pieces may be compared to a beam supported at two points and carrying a concentrated load. The deflection of such a beam is proportional to the third power of the span; and in the analogy the span corresponds to the wheel base of the truck. Therefore, the shorter the wheel base, the stiffer the truck side, and in view of its long wheel bases it is evident that the single truck is at a disadvantage in this respect. For instance, a truck side where the wheel base is four feet and six inches is 3.76 times stiffer than a truck side of equal section where the wheel base is seven feet.

TABLE I.

Comparison of the cost of wheels and brake shoes on Double and Single Truck Cars for the year 1897.

Average daily car mileage-115.

TABLE II.