| Type. | Cylinders. | Diameter of driving- wheel. | Weight of engine in working order, exclusive of tender | Weight of engine and tender without water or fuel. | Approximate price. | Price per pound. | |
| Diam. | Stroke. | Inches. | Pounds. | Pounds. | Cents. | ||
| "American" Passenger | 8 | 24 | 62 to 68 | 92,000 | 110,000 | $8,750 | 7.95 |
| "Mogul" Freight | 19 | 24 | 50 to 56 | 96,000 | 116,000 | 9,500 | 8.19 |
| "Ten-wheel" Freight | 19 | 24 | 0 to 58 | 100,000 | 118,000 | 9,750 | 8.26 |
| "Consolidation" Freight | 20 | 24 | 50 | 120,000 | 132,000 | 10,500 | 7.95 |
| "Decapod" Freight | 22 | 26 | 46 | 150,000 | 165,000 | 13,250 | 8.03 |
| Four-wheel Tank Switching | 15 | 24 | 50 | 58,000 | 47,000 | 5,500 | 11.70 |
| Six-wheel Switching, with tender | 18 | 24 | 50 | 84,000 | 98,000 | 8,500 | 8.89 |
| "Forney" N.Y. Elevated | 11 | 16 | 42 | 42,000 | 34,000 | 4,500 | 13.23 |
| Street-car Motor Locomotive | 10 | 14 | 35 | 22,000 | 18,000 | $3,500 to $4,000 according to design | 19.44 to 22.22 |
Fig. 33.—"Hudson" Tank Locomotive. By the Baldwin Locomotive Works.
The speed of locomotives, however, has not increased with their weight and size. There is a natural law which stands in the way of this. If we double the weight on the driving-wheels, the adhesion, and consequent capacity for drawing loads, is also doubled. Reasoning in an analogous way, it might be said that if we double the circumference of the wheels the distance that they will travel in one revolution, and consequently the speed of the engine, will be in like proportion. But, if this be done, it will require twice as much power to turn the large wheels as was needed for the small ones; and we then encounter the natural law that the resistance increases as the square of the speed, and probably at even a greater ratio at very high velocities. At 60 miles an hour the resistance of a train is four times as great as it is at 30 miles. That is, the pull on the draw-bar of the engine must be four times as great in the one case as it is in the other. But at 60 miles an hour this pull must be exerted for a given distance in half the time that it is at 30 miles, so that the amount of power exerted and steam generated in a given period of time must be eight times as great in the one case as in the other. This means that the capacity of the boiler, cylinders, and the other parts must be greater, with a corresponding addition to the weight of the machine. Obviously, if the weight per wheel is limited, we soon reach a point at which the size of the driving-wheels and other parts cannot be enlarged; which means that there is a certain proportion of wheels, cylinders, and boiler which will give a maximum speed.
The relative speed of trains here and in Europe has been the subject of a good deal of discussion and controversy. There appears to be very little difference in the speed of the fastest trains here and there; but there are more of them there than we have. From 48 to 53 miles an hour, including stops, is about the fastest time made by our regular trains on the summer time-tables.
When this rate of speed is compared with that of sixty or seventy miles an hour, which is not infrequent for short distances, there seems to be a great discrepancy. It must be kept in mind, though, that these high rates of speed are attained under very favorable conditions. That is, the track is straight and level, or perhaps descending, and unobstructed. In ordinary traffic it is never certain that the line is clear. A locomotive-runner must always be on the look-out for obstructions. Trains, ordinary vehicles, a fallen tree or rock, cows, and people may be in the way at any moment. Let anyone imagine himself in responsible charge of a locomotive and he will readily understand that, with the slightest suspicion that the line is not clear, he would slacken the speed as a precautionary measure. For this reason fast time on a railroad depends as much on having a good signal system to assure the locomotive-runners that the line is clear, as it does on the locomotives. If he is always liable to encounter, and must be on the look-out for, obstructions at frequent grade-crossings of common roads, or if he is not certain whether the train in front of him is out of his way or not, the locomotive-runner will be nervous and be almost sure to lose time. If the speed is to be increased on American railroads, the first steps should be to carry all streets and common roads either over or under the lines, have the lines well fenced, provide abundant side-tracks for trains, and adopt efficient systems of signals so that locomotive-runners can know whether the line is clear or not.
In what may be called the period of adolescence of railroads there was a very decided predilection on the part of locomotive engineers for large driving-wheels. Figure 34 represents one of the engines built as early as 1848 for the Camden & Amboy Railroad, with driving wheels 8 feet in diameter. Other engines with 6 and 7 feet wheels were not uncommon. In Europe many engines with very large wheels were made and are still in use. Here, as well as there, excessively large wheels have, however, been abandoned, and six feet in diameter is now about the limit of their size in this country.
Fig. 34.—Camden & Amboy Locomotive, 1848.