Much of the economy in the operation of pusher grades depends on the method of operation, which in turn depends on the method of their construction. When it is decided that pusher grades are necessary, the ideal method of construction is to concentrate the steep grades into one continuous rise. A turnout must be located very near the upper and lower ends of each pusher grade, so that the pusher-engine may be switched on and off the main track with a minimum of useless running. But the ideal arrange-. ment of a continuous grade is not always practicable. It sometimes becomes necessary to lay out a pusher grade for a length of perhaps three or four miles, and then, after a mile or two of comparatively level track, another pusher grade several miles in length must be added. It will usually be more economical to operate the entire distance as a continuous pusher grade, in spite of the fact that for a mile or two the pusher-engine is utterly unnecessary. It would be very difficult to so arrange the schedule of trains that each section of such a pusher grade could be operated separately with separate engines and keep the engines continuously employed. Economy in pusher-engine service demands that each pusher-engine shall be working as nearly continuously as possible. On account of the great loss of economy that occurs when two sections of a pusher grade are separated by a mile or two of comparatively level track, the engineer can profitably expend considerable study and even surveying, by his corps of men, in the endeavor to so modify the line that the total required difference of elevation can be condensed into a single grade.

Table XXXI. Balanced Grades For One, Two, And Three Engines

Basis: Through- and pusher-engines alike; consolidation type; total weight, 107 tons; weight on drivers, 53 tons; adhesion, 9/40, giving a tractive force for each engine of 23,850 lbs.; normal track resistance, 6 and 8 lbs. per ton.

Track resistance, 6 lbs

Track resistance, 8 lbs.

Through grade.

Net load for one engine in tons (2000 lbs).

Corresponding pusher grade for same ne load.

Net load for one engine in tons (2000 lbs.).

Corresponding pusher grade for tame net load.

4

One pusher.

Two pushers.

One pusher.

Two pushers.

Level.

3868 tons

0.28%

0.55%

2874 tons

0.37%

0.72%

0.10%

2874 "

0.47

0.82

2278 "

0.56

0.98

0.20

2278 "

0.66

1.08

1880 "

0.74

1.23

0.30

1880 "

0.84

1.33

1596 "

0.92

1.47

0.40

1596 "

1.02

1.57

1384 "

1.09

1.70

0.50

1384 "

1.19

1.80

1218 "

1.27

1.92

0.60

1218 "

1.37

2.02

1085 "

1.44

2.14

0.70

1085 "

1.54

2.24

977 "

1.60

2.36

0 80

977 "

1.70

2.46

887 "

1.77

2.56

0.90

887 "

1.87

2.66

810 "

1.93

2.76

1.00

810 "

2.03

2.86

745 "

2.09

2.96

1.10

745 "

2.19

3.06

688 "

2.24

3.15

1.20

688 "

2.34

3.25

638 "

2.40

3.33

1.30

638 "

2.50

3.43

594 "

2.55

3.51

1.40

594 "

2.65

3.61

555 "

2.70

3.68

1.50

555 "

2.80

3.78

521 "

2.85

3.85

1.60

521 "

2.95

3.95

489 "

2.99

4.02

1.70

489 "

3.09

4.12

461 "

3.13

4.17

1.80

461 "

3.23

4.27

435 "

3.27

4.33

1.90

435 "

3.37

4.43

411 "

3.42

4.49

2.00

411 "

3.52

4.59

390 "

3.55

4.63

2.10

390 "

3.65

4.73

370 "

3.68

4.78

2.20

370 "

3.78

4.88

352 "

3.81

4.92

2.30

352 "

3.91

5.02

335 "

3.94

5.05

2.40

335 "

4.04

5.15

319 "

4.07

5.19

2.50

319 "

4.17

5.29

304 "

4.20

5.32

It has been demonstrated elsewhere that the loss of energy incurred in stopping a heavy train is sufficient to run it along a level track for a mile or more. It is therefore desirable to couple and uncouple the pusher-engine without stopping the train if it is possible. The pusher-engine takes its name from the frequent custom of using the assistant engine literally as a pusher behind a freight-train, which enables it to accomplish its work without stopping the freight-train either at the top or bottom of the grade. For passenger service the assistant engine is always coupled ahead of the through engine, which means practically that the train must be stopped when the assistant engine is coupled on. The stop at the top of the grade is avoided by merely uncoupling the locomotive while running, and then running it ahead at increased speed on to a flying-switch, where a switchman is located so that the passenger-train passes the switch without stopping.

When the traffic of a road is very heavy a pusher grade may have several pusher-engines, whose sole duty is to serve the trains on that grade. Under such conditions they can usually be operated economically. When the train service is comparatively light, the pusher-engines are not so steadily employed, and the cost of the pusher-engine service is proportionally higher for each train assisted. If the pusher grade is located very near or even within a few miles of a large freight-yard, at which switching-engines are constantly employed, a considerable economy is frequently possible by employing the pusher-engines alternately as switching-engines in the yard and as pusher-engines on the pusher grade.

A still further economy is possible on roads of very light traffic, where the use of a pusher-engine would be a luxury. On such roads the passenger-trains are usually very short and light, and therefore are probably not affected materially by the rate of the ruling grade. On such roads also a delay of even 50% in the time of hauling a freight-train over the road is of comparatively small importance. In such cases the road can still be designed on the basis of pusher grades. The freight-train can be loaded up to the capacity of a single engine on all through grades which are less than the pusher grade. The freight-train can then be cut in two at the bottom of the pusher grade, and one half of the train can be taken up separately. The total engine mileage is no greater than with pusher-engine service, and in fact may be somewhat less, for reasons given in the next section. Almost the only objection is that due to the loss in time, and on a road of very light traffic this is a small matter. This method should not be forgotten, particularly in the design of light-traffic roads, since it has all the advantages of enabling a road to be designed, if necessary, on a pusher-engine basis, and yet if the traffic should ever increase, so that the delay, due to this method of operation, becomes objectionable, the normal method of pusher-engine service may be adopted. The engineer should not forget that the pusher-engine method must not be discarded simply because the road may not at first have an amount of traffic which would justify the ordinary method of pusher-engine service.