Steam loco variations

Discussion in 'Getting Started' started by BDC, Dec 3, 2001.

  1. BDC

    BDC Member

    What's the reasoning behind all the variations steam locos had with their lead and trailing trucks. (ie 2-6-2, 4-6-2, 2-6-4) I understand about the number of drivers and the need for power and traction, and that lead and trail trucks helped guide the loco but why the differences? If more axles on a truck is better, why not go with 2 or 3 instead of 1 axle? It shouldn't have added that much weight, should it?

    And why would one railroad but several versions of the same "base model?" For example, ATSF had 2-8-2 Mikados and 4-8-4 Northerns. Is it because that's what the manufacturer had in production?

    Just trying to expand my knowledge, which will eventually lead to an expanded inventory. Thanks!
  2. roryglasgow

    roryglasgow Active Member

    LONG ANSWER: (see below for short answer)

    The pilot trucks served to help guide the driving axles and keep the locomotive from derailing (especially on rough track and tight turns) and support the front end of the locomotive. Trailing trucks were usually added to designs to allow for a larger boiler and/or firebox. The exact number of pilot and trailing axles was determined by how much power the engine could produce and where the driving axles needed to be placed for the best traction. A well balanced locomotive design typically had the driving wheels near the middle, with enough pilot and trailing axles (if any) to support the remaining weight and keep the engine from derailing.

    As I understand it, pilot trucks made it a little more difficult to cross the points on switches. Yard engines typically had no pilot or trailing axles. (Although some mainline engines were retired to yard service.)

    Locomotive manufacturers were constantly trying to figure out how to make better locomotives. As technology improved, and railroads became more widespread and specialized, builders produced a wider variety of locomotives to fit their customer's specific needs. For example, Shays and Climaxes are really good at pulling heavy loads over poor track along steep grades; but 4-8-4 Northern and 2-8-2 Mikado locomotives are better for fast freight service on good mainline roads.

    A general rule of thumb is that in order to make a steam locomotive more powerful, you have to give it a bigger boiler (or boilers). Technological advances like superheating and better firebox designs improved engine efficiency, but there was always a need for something bigger and better. Builders tended to create new designs out of older ones, establishing a logical progression. For example:

    4-4-0 American - start here...
    4-6-0 Ten Wheeler - bigger boiler, added extra drive axle to get more power
    2-6-0 Mogul - Added extra drive axle to get better traction for steep grades and curves, took away extra pilot truck (engine wasn't much bigger overall)
    2-8-0 Consolidation - Improvement over 0-8-0 with more power than typical 2-6-0 and 4-6-0 designs.
    2-8-2 Mikado - Bigger firebox than 2-8-0, more powerful.
    4-8-4 Northern - Huge engine with even more power than a 2-8-2.
    ...and so on.

    (Manufacturers couldn't just jump to a larger engine size without figuring out first how to feed it enough fuel and keep enough steam coming to make the engine effective. Later technological improvements allowed for bigger engines...)

    So a railroad might go to a manufacturer and say, "We need a locomotive than can do such-and-such, and nothing exists that can do it." The manufacturer might then build a new model based on an old design (extend the boiler, add a drive axle, add extra pilot wheels and trailing axles to support the weight) and sell it to the requesting railroad. The new design then usually got named after the first railroad to request/buy it. Other railroads usually bought a particular design, too, if it fulfilled a need. ATSF most likely bought a lot of Mikados because it fulfilled their specific needs for power vs. cost over certain parts of their system. Other railroads that ran over different terrain and/or had different practices for moving freight bought other designs that suited their purposes better.

    And, one wheel configuration used by one railroad might differ from another railroad's due to modifications made to suit their specific needs. (A 2-8-0 owned by one railroad might differ slighly than a 2-8-0 used by another.)

    Keep in mind, too, that long haul freight trains often had several locomotives involved with getting the train from point A to point B. For example, a train that left a city in the flatlands might be pulled by an engine that was good at running fast on relatively flat track. When it got to the mountains, an engine that was more appropriate for mountain travel might be coupled on or replace the other locomotive.


    It all came down to adhesion and how fast they wanted to move...


    (Sorry for the long reply!)
  3. BDC

    BDC Member


    Thanks for the reply, Rory. I thought it might be something along those lines, but I really hadn't considered how technology played a role in loco design. Appreciate the time and effort you put into it. Again, thank you.
  4. Drew1125

    Drew1125 Active Member

    Just thought I'd point out that the Nashville, Chatanooga, & St. Louis (a.k.a. The Dixie Line) had 4-8-4's too, but a RR that ran through the heart of the Confederacy would never stoop to calling one of it's locomotives a "Northern"! :eek:
    They called 'em Dixies! :D
  5. roryglasgow

    roryglasgow Active Member

    True, different railroads would rename a particular wheel configuration to suit themselves. Check out They have a section on wheel classifications that lists the various names each configuration had...

  6. Bill Stone

    Bill Stone Member

    Great info up there.

    I would add that my understanding of the prime reason steam switchers did not have lead or trailing trucks was to give them the shortest wheel base possible, so they might be able to fit on a turntable (or a short spur) along with a car or dead loco they were moving. The extreme, of course, was switchers without tenders.

    Also, many early interurban steam locos, and logging locos had trailing trucks intended to lead them when running in reverse, as they seldom had the luxury of being turned, and ran in reverse with trains as much as forward.

  7. Gary Pfeil

    Gary Pfeil Active Member

    I think the primary reason switchers did not have lead and trailing trucks is that they had no need for them, considering the slow speeds st which they worked. Also, this kept 100% of the weight on drive wheels, improving tractive effort.

  8. jimmybeersa

    jimmybeersa Member

    Steam loco site

    Thats a great steam loco site thanks! Will be of gereral interrest I am sure. The Gauge is such a fantastic medium for
    finding out about all things on railroading, just pose a question and someone will come up with an answer, sure keeps this old brain of mine ticking over


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