Steam Power

Have you ever boiled a kettle of water for tea or cooked a pot of pasta on the stove? If you have, the power of boiling water and making steam was at your service. Heating water to its boiling point causes a change in the water’s state from a liquid to a gas or vapor. This vapor is steam and it has many remarkable qualities, the most prominent being its incredible potential to transmit power when handled in specific ways. The Mt. Tamalpais and Muir Woods Scenic Railway locomotive #9 harnessed the awesome power of steam to lift passengers over 2,500 ft to the summit of the mountain after which the railway was named. Let's strike a match and make some steam!


Exposed boiler on No. 9 during asbestos removal project. The boiler is usually hidden under a sheet metal cover.

A steam locomotive’s most crucial feature is its use of water and the ability to convert that water into pressurized steam. There are two water and steam related tanks on a steam locomotive known as the tender and the boiler. On Engine #9, the tender rides on the rear of the locomotive’s frame. Its outer shell forms the surface on which the beautiful gold leaf lettering spelling out “Mt. Tamalpais and Muir Woods” was painted. The tender tank has a water capacity of 850 gallons stored at “normal” atmospheric pressure and temperature. Water from the tender is pumped through a very clever device called an injector, a pump with no moving parts, into the boiler.


Until it passes through the injector, the water is just a very heavy dead weight that the locomotive has to drag around with it. Now is when the water goes to work. The boiler is a steel pressure vessel occupying the forward half of the locomotive. The boiler in #9 is oil-fired, and takes cool water from the tender and heats it to its boiling point to create steam. The boiler both produces the steam and contains it at a very high pressure reaching up to 180 psi. This stored steam energy has immense power. When the locomotive’s throttle is opened, a huge surge of energy is released into the locomotive’s “V-twin” steam cylinders located in the middle of the frame. This release of steam causes its volume to expand to around 1700 times what it was inside the boiler. This single fact alone is why steam locomotives work at all. The powerful steam pushes the cylinders up and down, and this motion eventually is transmitted through several cranks, shafts, and gears to create rotary motion that turns the locomotive’s wheels. That motion, in turn, moved the 36 ton engine and three fully loaded passenger cars up the steep rails of Mt. Tamalpais.


Does the sound “chugg chugg chugg” remind you of a train? That familiar sound signals the final stage of the steam’s journey through a locomotive. It is created when the steam is being shot up the smoke stack after it has been exhausted from the steam cylinders. The steam condenses as it escapes from the smoke stack and falls to the ground to water the trees, flowers, and poison oak (stay on the trails!).


Stay tuned for more articles like this one. We have lots of fun technical facts to explore on #9 and we want you to come along and enjoy it with us.

David Waterman

Mechanic on #9

FRIENDS OF NO. 9

info@friendsofno9.org (415) 302-7677

372 Bel Marin Keys Bl., Suite C-2, Novato, CA 94949

© FRIENDS OF NO.9