Thoughts From the Front Porch

Firebox and Drivewheels

Nothing bespeaks sheer power like a steam railroad locomotive.  This machine is alive even while sitting still; a real-life, fire-breathing dragon of unbelievable strength.  Steam engines do not struggle; they simply move with a force that can only be described as inexorable, absolute.

The most amazing thing about all this power is that it is produced by a pair of steel discs not much larger than automobile hubcaps.  How is such a thing possible?

Like any reciprocating engine, locomotives are driven by pistons which are housed within cylinders. You can easily spot these barrel-like objects, usually toward the front of the engine and connected to the main wheels with heavy steel bars.  Pistons & cylinders harness the pressure of expanding gases to generate their thrust; auto engines do it by burning fuel within the cylinders, whereas steam engines get their power from steam piped into the cylinders from a boiler.

A steam locomotive piston is typically a round disc, approximately eighteen inches to two feet in diameter.  Incredibly, this disc and its mate on the other side of the engine can produce thousands of horsepower and tens of thousands of pounds of “tractive force”.  How?  Well, like most engineering marvels, it’s all a matter of arithmetic.

Steam is powerful stuff.  It has an incredible ability to expand, exerting a huge amount of force from a relatively tiny amount of boiled water.  The hotter the steam, the more it can expand before cooling off.  Typical steam pressure in a locomotive can range anywhere from a hundred pounds per square inch to more than a thousand, depending on the engine’s design.  Most older engines operated at about 300PSI, so we’ll use that figure in our calculations.  Applying the pie-are-square rule, an 18″ piston has a total surface area of about 250 square inches.   If you push this piston with a force of 300PSI, you will get a total force of 75,000 pounds!  Remember, the pressure is figured PER SQUARE INCH.  And that’s just for one stroke of one cylinder; a steam locomotive has at least two, sometimes four cylinders.  Raise the steam pressure or widen the piston, and the power increases enormously.

Most locomotives have double-acting cylinders, so the steam pushes on both the forward and back stroke of the piston.  Also, they are timed so that at least one piston is always positioned to accept steam and produce tractive force.   Some are designed with separate high- and low-pressure cylinders so that steam that has exhausted most of its expansive power is then piped into a much larger cylinder with more square inches of surface area.

The hardest part of pulling a train is getting it started in the first place.  There may be millions of pounds of freight in a hundred or more railcars.  The engine itself can easily weigh more than a half-million pounds.  That’s when an EXTERNAL COMBUSTION engine really shines; it develops full power from the start.  Your car’s INTERNAL combustion engine must burn fuel within the cylinder to produce pressure that will gradually increase until it has enough force to move the piston.  Not so with an engine whose fuel is burned and pressure developed outside the cylinder.

Steam is most powerful when the locomotive is moving slowest.  It exerts maximum expansion when it can make the piston travel its full length before the spent steam is exhausted to the smokestack.

Once a locomotive gets up to speed, this power stroke is shortened a bit and the steam exhausted a little sooner to allow the piston to reciprocate more freely as it nears the end of its stroke.

Here’s a nice website that explains steam power in more detail: More.

When standing beside a steamer in a rail yard, you can hear the rumble and feel the heat from its firebox as puffs of live steam whoosh through the smokestack to maintain a draft for the fire inside.

Just before the train departs the station, a shot of live steam blows condensed water from its cylinders, which erupts at waist level as a hissing blast of white fog.  This is to keep the cylinders from being damaged because water, unlike steam, is not compressible.  As the engine begins to move, exhaust steam from its cylinders blows directly into the stack to produce that familiar choo-choo sound.

As I said in the opening paragraph, steam power is absolute; if the load is too heavy or the locomotive too light, its drive wheels will simply spin in place until friction improves enough to allow them to grip the rails.

Double Engine

One trick used by train engineers is to first move the train in reverse, which pushes all the slack out of the couplings between railcars.  If you’ve been around trains much, you’ve heard the machine-gun-like banging of cars being shoved against one another in this maneuver.  Then, as the train begins to move forward it only has to move one car at a time until the whole string is moving.

A steam locomotive in passing makes the ground rumble like a small earthquake.  Passengers enjoy a super-quiet, steady ride not possible in a car, bus or airplane. There’s nothing quite like it, whether you are standing beside the tracks or comfortably seated in a wide, softly upholstered reclining armchair with no seat belts.  My father once sat a while with me on a steam train I had boarded for a trip to Collinsville, AL.  It’s departure was so smooth that Dad didn’t even realize it was moving until he looked out the window. They stopped the train to let him get off, and he had to walk several blocks back to the station.

I was privileged to have ridden behind several steam locomotives as a child, and again as an adult on steam excursions sponsored by a local railroad club.  There are still a few places where one can experience this vintage thrill for themselves.  Here’s a couple of them:

Link 1

Link 2.

Don’t miss out on this, folks.  Once all these goliaths are scrapped, there will never be anything like them again.

Views From Benny Hill is a series by Jerry Smith

Views From Benny Hill