The Atmospheric Railway
For those unfamiliar with the Atmospheric Railway it was a scheme in which pumping stations a 3 mile intervals pulled vacuum on a 15 inch pipe. A slot was cut in the pipe so that a piston might attach to the piston car (engine) thru it. The pipe was sealed by a valve running it’s entire 3 mile length. The valve was effectively a continuous length of spliced and glued leather. The valve was forced into a seat of tallow and wax at first by the weight of iron backing and as the vacuum rose by the pressure difference. All in all the valve formed an amazingly good seal. The entire length of the pipe only leaked approximately as much as a 3 ½ inch diameter hole. While this may sound large it represented a hole only 0.030 inches in diameter for each foot of pipe.
The pipe was evacuated with a 5’ 7” x 5’ 6” pump making 50 strokes a minute, sweeping 113 cubic feet of volume per second. In normal operation the pipe was exhausted to 20 inches Hg vacuum before a train was put on the pipe. Except at Exeter St Thomas where trains stopped on the pipe the train was typically started from a platform siding without a pipe, this was accomplished by an auxiliary pipe containing a piston with a cable attached. The train was pulled thru points lined to the pipe with the cable which was dropped by pulling a pin before entering the pipe. Samuada, inventor of the Atmospheric Railway, had a patent valve which would open as soon as a train entered the pipe or a valve man may have opened the run-thru valve by hand. In either event as soon as the valve was opened the train would accelerate quickly to something near 40 mph and then run at almost steady speed for the remaining 2 miles of pipe and then brake sharply for the last half mile.
The sharp acceleration was possible because 20 inches of vacuum applied about 1760 pounds of tractive effort to the piston. For approximately the first half mile the train accelerated into stored vacuum pumped before the train entered the pipe. Once the train filled the stored vacuum it’s speed depended on the amount of vacuum needed to balance train resistance. As the vacuum increases so does the tractive effort but the thinner air is drawn more slowly down the pipe, to illustrate: At 50 strokes per minute and zero vacuum(open pipe) there is zero tractive effort and the speed of the air down the pipe is 63 mph. At 20 inches Hg the pump is just able to keep up with leakage without the train moving but it produces 1760 pounds tractive effort. The following table offers a rough idea of relations of Vacuum, Air Speed in the pipe, Tractive Effort and Train Resistance at the speed of the air in the pipe. As may be seen the balancing point, on level track, falls between 5-10 inches Hg and 42-52 mph for the classic 3 carriage train.
|
VACUUM |
AIR SPEED |
TRACTIVE EFFORT |
TRAIN RESISTANCE AT AIR SPEED |
|
0 |
63 |
0 |
612 |
|
5 |
52 |
441 |
489 |
|
10 |
42 |
882 |
389 |
|
15 |
31 |
1323 |
311 |
|
20 |
21 |
1760 |
255 |
With a 50 Ton goods train in tow the balancing point comes at 10-15 inches Hg at 31-42 mph.
|
VACUUM |
AIR SPEED |
TRACTIVE EFFORT |
TRAIN RESISTANCE AT AIR SPEED |
|
0 |
63 |
0 |
1750 |
|
5 |
52 |
441 |
1398 |
|
10 |
42 |
882 |
1111 |
|
15 |
31 |
1323 |
887 |
|
20 |
21 |
1760 |
728 |
Braking is likewise nimble. On
locomotive drawn trains of the day it was not unusual for the engine and tender
to weigh as much as the coaches, and thus adsorb half or more of the braking
effort. I’ve estimated the weight of a
3 carriage Atmospheric Train at 19 ½ Tons.
5.5 Tons Tare for each carriage and 6.5 Tons Tare for the Piston
Carriage with 2
Tons of passengers. From
So we see both the appeal and the downfall of the Atmospheric. Had it worked it would have made a fine stopper, but with an average of only .030 diameter hole allowed per foot of pipe a stick or pebble or rat hole in the leather could easily be a disabling leak.