The Huge Engine

Watch this short animation.  Or, this one.  All steam locomotives, regardless of the era or the size, worked pretty much the same way: fuel (wood, coal, oil) makes heat, which boils water under pressure, which turns to steam, which enters the cylinders, which pushes the locomotive, which pulls the train (which is why they exist).  But, like all technology, there were many incremental improvements over time.   The basic goal of the locomotive designer was to maximize efficiency while minimizing operating and maintenance costs; and the best locomotives found a winning balance between the two.  

It depended on how much it was pulling (the weight of the train) and what kind of grade (incline) it was climbing.  In normal service, these types of locomotives could move a (roughly) 9,000 ton freight train at 50-mph on level track, and a passenger trains of (about) 1,500 tons at 70-mph (maximum allowed speed, not theoretical speed).   However, going up a 2.5% grade (meaning a 2.5 foot elevation increase over 100 feet of track, which is the grade over Donner Pass) the locomotive could pull (roughly) 2,000 tons but only go (about) 20 mph; which is why helper engines were used on mountain grades to allow for heavier trains.  A steam locomotive was generally designed for specific purposes: fast freight trains, slow freight trains, very slow freight trains, high-speed passenger trains (say, over 80 mph) and medium-speed passenger trains (say, less than 80 mph).  The primary difference in design was the diameter of the driving wheels (aka: drivers).  Imagine an automobile - with a (broken) manual transmission - having a single gear: an automobile with only a low-gear (small drivers) is easy to get moving due to high torque, but is slow; an automobile with only a high-gear (large drivers) is fast, but has low torque so is difficult to get moving.  This was one of the many design deficiencies compared to diesel locomotives, which were pretty much above-average at everything, with no - or very little - modifications; primarily because diesel locomotives have a "transmission like device" which converts horsepower into torque.  Also note that a steam locomotive not pulling anything could easily travel so fast that it would literally self-destruct due the wheels being out of balance and the "value gear" parts, which weighed tons, needing to change directions many times a second at high speeds.  The maximum speed was limited only by the rulebook and nerve (aka, lack of common sense) of the engineer (and, as all engineers then were males, that only leaves "rules").  

They sure did.  However, it wasn't very common after government regulations were put in place in the early 20th Century.  See the next FAQ for how it could happen.  

There was (roughly) 10,000 gallons of water in the boiler.   The amount of water in the boiler, however, wasn't constant.   Depending on power needs, the engine crew would intentionally allow the water level (water amount) to vary by a thousand gallons or so.   However, if the water level goes below a specific amount, the top of the firebox is no-longer covered with water and the locomotive will exploded.  

The estimate of water in the boiler is based on the weight of water and the difference in the locomotive's weight of 657,900 lbs vs an empty weight of 576,000 lbs.   

There was 22,000 gallons of water and 6,100 gallons of Bunker C Fuel Oil.   A normal run from Roseville yard (east of Sacramento) to the top of Donner Summit required 3 or 4 stops for water.   Imagine a train of 100 refrigerator cars - about 5,000 tons - with 3 Cab Forwards:  one at the front, one about half-way back in the train, and one at the rear.  At 3 or 4 places going up the hill, the train had to be stopped and each locomotive tender refilled with water.  At each water stop, the train had to started and stopped three times to refill each tender.    This was done without radios, in the middle of winter storms, in the blackness of night.   Different times...

The AC-12 class produced about 6,000 hp at 40 mph, as measured at the rear coupler of the tender (this is called drawbar horsepower).   For comparison, the diesel locomotive that is generally credited with "overthrowing the reign of the steam locomotive", called the F-3 and built by General Motors, only had 1,500 hp.  So, steam engines were very powerful machines, but horsepower isn't the only way to measure locomotive performance.   The other common measurement - and, actually, the more preferred one, is tractive-effort: which is how much the engine can pull.   The AC-12 class could exert about 124,000 lbs of force (or 62 tons).  Hold on a minute, you might say: trains weighed more than 62 tons. How can it get a train started?  Ahh,  but don't forget: the rolling resistance of a railroad car is only (about) 1% of its weight; so a 62 ton pulling force can move a 6,200 ton train (roughly).  That's one advantage that trains have over most other forms of transportation (except things that float), there's not much resistance to moving them.  How about the F-3, how much did it pull?   About 55,000 lbs (or 27.5 tons), but that's only at 0 mph, then it drops off rapidly.  Steam locomotives are constant pulling machines; the same pulling force at all speeds.  So, this means horsepower (work over time) increases with speed.  Diesel locomotives, however, are constant horsepower machines; the same horsepower at all speeds.  But this means pulling force decreases with speed.  See this link for an engineering-geek quality explanation of steam vs diesel locomotives.

No, all the Cab Forward locomotives burned Bunker C fuel oil, and "Rosie" is holding a giant wrench not a coal shovel.  In addition, if "Rosie" was standing there when the engine went through a tunnel she'd have been killed (see: "Monkey Deck" paragraph in this link).  And, technically, that's not "Rosie", as "Rosie the Riveter" represented the women who worked in the Defense Industry during 2nd World War.  The Museum's "Rosie" represents the thousands of women who worked for railroads during the 2nd World War when many of the male railroad workers served in the armed forces.   She is specifically a locomotive maintenance worker, and the steam locomotives needed lots of maintenance.

It couldn't, which is why the Cab Forward design wouldn't have been possible with a coal or wood burning locomotive.  On coal and wood burning locomotives the tender (the fuel/water car) must be next to the firebox (where the fire is).   A coal burning Cab Forward would have needed to push the tender, which would have made forward vision difficult, which is why they burn Bunker C Fuel Oil instead.

Most steam locomotives have a firetube boiler.  A firetube (aka boiler tube in this link, or a flue, #28) is a pipe that carries the exhaust gasses (aka smoke) from the firebox (#27) through the hot water in the boiler to the smokebox  (#11).   There are many of these firetubes running from the firebox  (visualize a coke can filled with drinking straws).  The firetubes help transfer more of the fire's heat into the water.  

This is called the "valve gear" (see #19 - #25), which is used to transmit power from the steam cylinders (#24, of which there are two on each side) to the driving wheels (#43, the large wheels of which there are four per cylinder).  Just as important, the value gear creates the motion used to move the valves (#25) which control the movement of steam into and out of the cylinders.  This link shows an animation of how it all works (best viewed landscape).