The Tom Thumb and the Road to Ellicott’s Mills - Part 2
Updated: Oct 9
PART 2 OF 4 · HISTORY AT ELLICOTT CITY STATION
Building and Testing an Experimental Locomotive
Improvised parts, uncertain dimensions, broken wheels, missing copper, and repeated trial runs reveal what the Tom Thumb really was .. a working locomotive prototype built in iron and steam.
Built for an answer, not for regular service
Baltimore imagined the journey west before Peter Cooper had a locomotive ready to attempt it.
A song associated with the B&O’s 1828 groundbreaking pictured travelers who could “go to bed in Baltimore, and breakfast in Ohio.” The words captured the city’s ambition. They did not describe anything the railroad could yet deliver.
By May 1830, the B&O had opened thirteen miles of track from Baltimore to Ellicott’s Mills. Horses pulled the cars. Passengers came in large numbers. The service produced revenue.
The railroad had proved that people would ride.
It had not proved what would carry them west.
That was the question waiting at Mount Clare.
The B&O’s facilities there were still young. This was not yet the vast industrial complex that would later build and repair locomotives by the hundreds. The early site served a horse-powered railroad. It included tracks, car storage, stables, and basic repair facilities.
James Maccubbin Carroll, a distant cousin of Charles Carroll of Carrollton, had provided land and right-of-way for the railroad near his Mount Clare estate. The grounds expanded as B&O operations grew, although surviving accounts differ over the dates, acreage, and precise locations of some of the earliest buildings.
That uncertainty matters less than the larger fact. Cooper was not entering an established locomotive factory.
There was no American locomotive industry waiting to fill his order. There was no standard B&O locomotive design to copy. There was no shelf of proven parts.
There was a shop, a railroad, and a problem.
Cooper later called the workplace a coachmaker’s shop. There he combined a small steam engine brought from New York with a vertical boiler made in Baltimore, a platform, some railroad wheels, and the mechanical connections needed to turn the assembly into a railway vehicle.
The engine was not the locomotive. It was one part of it.
The boiler produced steam. The steam moved a piston. The mechanism transferred that motion to the wheels. The platform held the parts together. The frame and axles had to keep them aligned while the machine traveled over imperfect track.
Every connection mattered.
Cooper was not assembling a locomotive from an approved plan. He was building an argument out of iron, steam, and whatever useful materials he could obtain.
The machine we cannot reconstruct exactly
No complete original construction drawing of the Tom Thumb is known to survive.
That absence has consequences. Later writers often describe the locomotive as though every measurement had been preserved. It had not.
J. Snowden Bell reproduced technical information describing a single cylinder approximately three and one-half inches in diameter and road wheels about thirty inches across. John H. B. Latrobe later gave the cylinder as three and one-quarter inches in diameter with a fourteen-and-one-quarter-inch stroke. He also described road wheels two and one-half feet in diameter.
Those figures are close enough to establish scale. They are not identical enough to justify false precision.
William H. Brown’s 1871 history explains why. Brown published a sketch prepared from Latrobe’s recollections and submitted to Ross Winans for review. Winans accepted its general features as substantially correct. He also warned that many details were conjectural or defective.
The number, length, and exact size of the boiler tubes were unknown.
This was memory put on paper, not a shop drawing recovered from 1830.
Lawrence W. Sagle returned to the problem in 1948. He compared Brown’s published drawing with Joseph G. Pangborn’s full-size wooden model made for the 1893 World’s Columbian Exposition and the operating replica constructed for the B&O’s 1927 Fair of the Iron Horse.
The comparison revealed substantial differences.
Sagle estimated the original boiler at about twenty-one inches in diameter and approximately five feet high, including the firebox. Pangborn’s model and the 1927 replica used a larger boiler. The later operating replica also had a much larger cylinder and stood considerably higher above the rails.
The original may have weighed less than one ton. The operating replica weighed about five tons.
These differences do not mean the replicas are without value. They mean the replicas must be understood to be later interpretations designed for exhibition and, in the case of the 1927 version, reliable operation.
They help us picture the Tom Thumb.
They do not settle every question about it.
The boiler at the center of the experimental steam engine
The boiler was one of the largest recorded expenses in the project.
Edward C. Mack cited summer 1830 charges totaling $171.92, including $109.60 for the boiler. That single item accounted for most of the documented construction cost he identified.
The boiler stood upright. A fire burned in its lower section. Heat passed through small tubes surrounded by water. As the water boiled, steam collected and supplied the cylinder.
Later accounts say Cooper used iron musket barrels as boiler tubes because suitably small manufactured tubing was difficult to obtain.
It is a memorable detail. It is also one that needs care.
The gun-barrel account comes from Cooper’s later recollections. No surviving bill or construction drawing provides a complete list of the tubes used in 1830. The tradition illustrates the difficulty of obtaining suitable small tubing and the improvised nature of the project. It does not give us a dependable parts inventory.
What is certain is the problem Cooper faced.
The boiler had to be compact enough for a tiny locomotive, yet productive enough to keep the cylinder working. A weak fire would make steam too slowly. A large boiler would add weight. Too little steam pressure would leave the engine powerless. Too much could place the experimental apparatus, and everyone riding or standing nearby, in danger.
The machine had to be small.
Its task was not.
How the steam engine produced motion
Steam entered a single vertical cylinder and pushed a piston back and forth.
That movement had to become rotation.
The familiar solution was a crank. A connecting rod linked the piston to a crankshaft, turning the piston’s straight motion into circular motion. Gears then carried that rotation to the road wheels.
But that was not Cooper’s first arrangement.
He had already patented a different method of converting the piston’s motion into rotary power. In a detailed 1875 recollection preserved by Sagle, Cooper described chains, wheels, catches, and alternating motion. The arrangement was meant to avoid what he believed was a loss of power through an ordinary crank.
It worked .. until it did not.
As the working surfaces wore, the mechanism began to slip under strain. Cooper abandoned it and substituted a crank and gearing.
That change is central to the story.
Cooper had invested thought and pride in his own mechanism. Yet the purpose of the Baltimore experiment was larger than the mechanism itself. He was trying to prove that a locomotive could operate on the B&O.
When one idea endangered that objective, he let it go.
The design changed. The goal did not.
Gearing for speed
The locomotive’s wheels were small, so gearing helped them turn rapidly.
A large gear on the crankshaft engaged a smaller gear connected with the forward road wheels. The arrangement increased wheel speed. It did not create power. Instead, it exchanged turning force for faster rotation.
That distinction helps explain the Tom Thumb’s unusual character.
The little engine could move at a surprising speed when conditions were favorable. That did not mean it could haul the heavy loads expected of a regular service locomotive.
Speed was one question.
Pulling power was another.
This was not a minor detail. An engine could produce steam. The piston could move. The crankshaft could turn. Yet the locomotive would still fail if the gears shifted apart or slipped out of mesh.
The gearing warrants more than just a brief mention. According to the company report reproduced by Bell, it was employed to achieve speed with the small, thirty-inch wheels. It altered the relationship between the engine's rotation and the wheels' rotation, without generating extra power. Consequently, a locomotive could spin its wheels rapidly yet remain too small to pull the loads needed for regular service.
The gears also had to remain aligned. Brown’s account says the axle carrying the smaller gear was restrained against sideways and lengthwise movement so the teeth would remain properly engaged. In practical terms, the teeth had to stay in mesh while the vehicle traveled.
The frame and axle supports were therefore part of the power-transmission system, not merely a platform beneath the boiler. An engine could produce motion, yet fail to deliver it reliably if the parts connecting it to the wheels shifted out of position.
The frame was more than a platform.
It was part of the power system.
I retouched several of the more notable public domain illustrations based on memories of what the Tom Thumb might have appeared like. They vary slightly in aspects such as the coal storage, the location of the coal "fire box" door, the fan blower, the tubes, and the water storage. Some elements may have been deliberately left out.
The blower that kept the fire alive
Keeping the fire working was another problem. A boiler that small could not depend on coal and natural air draft alone.
Later descriptions identify the fuel as anthracite coal.
So Cooper added a mechanical blower. A wooden drum attached to one of the road wheels drove a belt or cord connected to a pulley on the blower shaft. As the locomotive moved, the blower forced air through the fire.
More air strengthened combustion. Stronger combustion produced more heat. More heat produced more steam.
The arrangement formed a chain of dependence.
The locomotive needed steam to move. Movement drove the blower. The blower strengthened the fire. The fire produced the steam needed to keep moving.
As long as the chain held, the system worked.
If the blower belt slipped, the fire weakened. Steam pressure fell. The engine lost power even though its cylinder and gearing remained intact.
This mechanical weakness later became central to the horse-race story. But before it became legend, it was an ordinary engineering problem: a small boiler struggling to supply enough steam for sustained operation.
Later drawings and replicas differ in the size and placement of the blower. Sagle believed the original blower was smaller than those shown on the Pangborn model and 1927 operating replica. He also concluded that it was associated with the smokestack rather than resting prominently on the platform as some reconstructions suggest.
Its precise appearance remains uncertain. Its function does not.
The blower helped the small boiler produce steam fast enough to keep the locomotive moving.
Operating the Tom Thumb steam locomotive required mutli-tasking
The October 1897 Book of the Royal Blue described Cooper serving as both engineer and fireman. Although written many years after the experiment, the account helps explain the burden placed on him.
Operating the locomotive and maintaining its source of power were different jobs. Cooper had to control the machine while watching the fire, steam pressure, water supply, blower, belt, gearing, and track ahead.
Making steam was not a task completed before departure.
It had to continue throughout the run.
Compactness was part of the answer
The B&O’s curves were not incidental to Cooper’s experiment. They were one of the main reasons he built the locomotive.
Steam engines already existed. British locomotives had shown that steam could move trains. Cooper did not need to prove that locomotive power worked in the abstract.
He needed to prove that it could work here.
The B&O had built a railroad through uneven Maryland terrain. Its route included grades and sharp curves, some chosen to avoid the cost of deeper cuts and heavier construction. Critics questioned whether locomotives based on British practice could follow that alignment.
Cooper’s answer was a compact machine with a short wheelbase.
The Tom Thumb was not designed for an ideal railroad. It was designed to confront the railroad that already existed.
Could it follow the curves?
Could it climb the grades?
Could it carry people over the line rather than merely move itself inside a shop?
Those questions would be answered one trial at a time.
Tom Thumb mishaps, thefts and delays remembered half a century later
Cooper’s later accounts describe a series of setbacks. Someone reportedly operated the machine without understanding its mechanism and broke part of a wheel. A replacement wheel was damaged during finishing. Copper steam pipes disappeared shortly before a planned trial, apparently stolen for their scrap value.
Cooper said the missing copper delayed the work for about a week.
The stories are vivid. Their sequence is not secure.
Mack compared Cooper’s different recollections and found that he placed the wheel damage and missing pipes in different orders.
Cooper reported repeated damage, missing components, repairs, and delay while preparing the locomotive for a longer run.
These were not amusing side stories added merely for color. Each failure exposed a different weakness in the project.
A broken wheel made the entire locomotive unusable, no matter how well the boiler produced steam.
Missing pipes left the boiler and cylinder disconnected, no matter how carefully each had been made.
A slipping drive mechanism could produce motion without delivering it reliably to the rails.
An experimental machine only when all of them work together.
Preliminary movement and a railway trial
The surviving accounts indicate that the Tom Thumb was tested in stages.
Latrobe remembered the first raising of steam inside the Mount Clare carhouse. George Brown, the B&O’s treasurer; Alexander Brown, his father; B&O president Philip E. Thomas; and one or two other men watched Cooper open the throttle.
Steam entered the cylinder. The experimental drive began working with a clacking sound. The machine moved slowly forward, carrying several bystanders who had stepped aboard.
It was a small movement but its meaning was larger.
The separate components were acting as one machine. The boiler produced steam. The piston moved. The mechanism turned. The wheels carried the locomotive forward with people aboard.
That did not prove the Tom Thumb could travel thirteen miles.
It proved there was something worth testing outside the shop.
Cooper later recalled a preliminary movement on the railroad. In his 1882 letter to the Boston Herald, he described raising steam on a Saturday and traveling two or three miles with the railroad’s president and several other gentlemen aboard.
In the 1875 account reproduced by Sagle, Cooper remembered the event more cautiously. The president and two other men stepped onto the locomotive, and the party went “out a little way and came back.”
The two recollections differ in distance and detail. Neither provides a secure date or identifies the turning point. They may describe the same preliminary trial, or they may preserve overlapping memories of more than one short movement.
What they do show is a logical progression.
First, the locomotive moved inside the carhouse.
Then it moved onto the railroad.
Next came a test with a greater load and a more demanding distance.
A larger passenger demonstration
Cooper described a later run with six people aboard the locomotive and thirty-six in an attached passenger car. He recalled an outward journey of one hour and twelve minutes and a return of fifty-seven minutes.
These figures come from recollections recorded decades after 1830. They should not be merged automatically with the B&O’s account of the documented August 28th journey, which gives different passenger totals.
The differences may reflect separate demonstrations. They may reflect different ways of counting riders. They may also show how recollections changed over time.
Until the accounts can be reconciled, each set of figures must remain attached to its source.
The important point is not lost.
Cooper was no longer demonstrating that his machine could move only itself. He was placing people aboard the locomotive and coupling it to a passenger car.
The load changed the test.
A machine moving alone could prove that its mechanism functioned. Pulling a car filled with passengers required more steam, greater traction, stronger connections, and sustained operation.
Reaching Ellicott’s Mills would demand more still. The locomotive would have to keep producing power over distance while following the B&O’s curves and climbing its grades.
These were practical stages of experimentation, even though no formal test program survives.
Riders, but not regular steam service
The people carried by the Tom Thumb were demonstration participants.
They were not ordinary customers in regularly scheduled steam service. That distinction is important.
The B&O had begun regular horse-powered passenger service between Baltimore and Ellicott’s Mills in May 1830. Cooper’s locomotive entered that working railroad as an experiment.
The B&O did not adopt it as a regular passenger locomotive.
Nor had Cooper built it for that purpose.
The Tom Thumb was too small, too lightly constructed, and too experimental to become the railroad’s standard source of power. Cooper did not need to persuade the directors to reproduce this particular machine.
He needed to show them what a larger locomotive might accomplish.
The riders gave the experiment an audience. A device moving inside a carhouse was interesting. A locomotive carrying people on the railroad created evidence that company leaders could see, feel, and judge for themselves.
From workshop device to public evidence
Each successful movement strengthened the Tom Thumb as an argument.
Inside the Mount Clare carhouse, the machine showed that its parts could work together.
On a short stretch of railroad, it showed that the assembled locomotive could move beyond the shop.
With passengers aboard, it showed that it could carry more than its own weight.
Each step placed greater demands on the same small machine.
The evidence does not preserve a complete calendar of this work. We cannot assign every broken wheel, missing pipe, mechanical change, and preliminary movement to a particular day.
But the sequence of development is visible.
Assembly.
First steam.
Movement.
Failure.
Repair.
Modification.
Another attempt.
The Tom Thumb was not the product of one uninterrupted act of invention. It emerged from a process of finding weaknesses and correcting them.
Each difficulty exposed another condition the machine had to meet.
Each repair moved the experiment closer to the railroad.
What success would, and would not, mean
The distinction between an experimental locomotive and a service locomotive remained fundamental.
Cooper did not need to show that the Tom Thumb could operate every day. He did not need to prove that it could haul the loads required of a mature railroad. He did not need to establish that its parts would withstand years of use.
Those questions belonged to the next stage of locomotive development.
His immediate task was narrower.
Could steam traction negotiate the B&O’s existing curves and grades while carrying a useful load?
If the answer was yes, the railroad could begin considering larger, stronger, and more practical locomotives. Reliability, capacity, durability, and economical daily operation could then become engineering requirements rather than reasons to reject locomotive power altogether.
The account reproduced by Brown closes on that provisional point. The work remained experimental, and further improvements had already suggested themselves to the inventor.
A successful test did not complete the engineering.
It revealed what the next machine would need.
A machine ready to make its argument
The Tom Thumb’s development was a process, not a single heroic moment.
Cooper began with a small engine rather than a complete locomotive. At Mount Clare, he combined it with a boiler, platform, railroad wheels, transmission, blower, and the connections needed to make the parts work together.
At last, the Tom Thumb became more than a workshop device. It became a working locomotive prototype, still temporary, still limited, but capable of carrying people on the B&O’s rails.
Now it had to make the journey that would determine what that movement meant.
On August 28, 1830, the Tom Thumb would leave Mount Clare for Ellicott’s Mills.
The Tom Thumb and the Road to Ellicott’s Mills SeriesPART 1 OF 4 Why Peter Cooper Built the Tom Thumb LocomotivePART 2 OF 4 Building and Testing an Experimental LocomotivePART 3 OF 4 The Journey to Ellicott’s Mills That Proved Steam Could WorkPART 4 OF 4 The Race, the Next Challenge, and the Making of a LegendBONUS The Tom Thumb Replica Returns to Ellicott City After 23 Years in Storage |
Research note
This article separates contemporary company evidence from later recollection and tradition. Numbers and dates are attributed where the surviving sources disagree.
Edward C. Mack, Peter Cooper: Citizen of New York (1949), chap. 7, pp. 107–121 and notes pp. 398–400; construction bills and comparison of Cooper’s recollections.
Peter Cooper, letter to the Boston Herald (1882), RHR-BL170; shop details, preliminary movement, setbacks, passenger load, and timings. Retrospective testimony.
J. Snowden Bell, The Early Motive Power of the Baltimore and Ohio Railroad (1912), pp. 6–8, company measurements and Latrobe’s technical description endorsed by Winans.
Angus Sinclair, Development of the Locomotive Engine (1907), later technical synthesis, dependent on retrospective accounts.
C. Edwards Lester, Life and Character of Peter Cooper (1883), pp. 19–21, small-scale demonstration purpose and 1830 construction date. Its claim that the original survived is not adopted.
Book of the Royal Blue, October 1897, “The Evolution of the Locomotive” (PDF p. 15); later account of gearing, anthracite, fan drive, and Cooper’s operating role. Its disputed dates and race narrative are not adopted.
Book of the Royal Blue, July 1898, “Song for the Day,” p. 23; reproduced celebration song associated with the 1828 groundbreaking.
William H. Brown, The History of the First Locomotives in America (1871), chap. XXI, “Peter Cooper Locomotive”; Latrobe–Winans correspondence, retrospective sketch, crank substitution, gearing, blower drive, and reproduced operating account. Accessed through the Hopkin Thomas Project transcription. Its 1829 narrative is not adopted over Mack’s bill-based analysis. Read chapter XXI
Hopkin Thomas Project, “Peter Cooper’s Locomotive, Tom Thumb, 1830”; compilation juxtaposing Brown and later historical material. Useful for comparison, not counted as independent confirmation of the passages it reproduces. Read compilation.
Mount Clare Museum website. https://www.mountclare.org/historic-site/after-carrolls/b-o-railroad
