kcsivils Wrote:
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> I am curious as to what engineering factors limit
> speed on narrow gauge railroads.
>
> What factors in regards to track limits speed?
Rail weight, tie spacing, quality of ballasting, drainage, frequency of maintenance--it all plays a role, and the narrower the gauge, the more it matters. Narrow gauge railroads in the United Stats were not typically known for the quality of their trackwork, and speeds were usually low.
>
> Does wheel size of the rolling stock limit speed?
In theory it does, especially in the days of journal bearings, although in practice actual top speeds are usually limited by factors other than this.
>
> What factors about locomotive design limit speed?
>
Lots. In a steam locomotive you need more firebox and boiler to create power. You want superheating to further increase that power. You need drive wheels large enough to contain sufficient counter-balance for the piston thrusts (this is part of why K-36 and 37 types are slow--inadequate balancing for high-speed running, even though they have plenty of power to theoretically do so). You need a practical center of gravity, and not too excessive a width relative to wheel spacing.
With respect to wheel size, Steam locomotive driving wheels directly affect the rotational rate of the engine itself. Steam engines don't like high-speed running, and even the very best modern steam locomotives (like the N&W J's) would come apart if you tried to run them at more than about 450ish RPM for any length of time (yes, I know of the PRR trial of a J that ran to 110 MPH, exceeding 500 RPM...they ended up breaking the locomotive). For 19th century practice, more like 300-350 RPM was the practical limit for daily use, and more like 100-200 RPM was preferred for ordinary economical cruising. 350 RPM on Dan Markoff's "Eureka" works out to just over 40 miles an hour, which happens to be the speed Baldwin guaranteed that model at in its advertising. UP 844 with its 80 inch drivers would be doing 450 RPM at a little over 100 MPH, which again was about the practical limit for regular operation. Sure it had the power to run 120+, but at the cost of high wear and threatening to break things. (the famous LNER "Mallard", the steam speed record holder, did indeed break itself during its record-setting run).
Note that you still have to have the power to achieve high speeds: Big wheels, by themselves, only allow the potential for speed, not the actual fact. The famous NYC 999 never even came close to the ridiculous 112 MPH it's sometimes credited for (by an untrained newspaper reporter, mind you) because it lacked sufficient horsepower to do so. Actual top speed with its train was in the mid 80's, as confirmed by the NYC on subsequent trials when they sought to confirm or refute the questionable claims.
On top of that, how much load are you pulling? The "Eureka" that Mr. Markoff mentions above could do fifty miles an hour or more given utterly perfect circumstances, but it could only make that speed pulling a couple of lightweight 19th century wood coaches. "Mallard" made its record (at the cost of breaking itself) running a train of only a couple hundred tons--and downhill. UP 844 was expected to be able to haul several times that, a thousand tons or more, at 90-100 MPH on level ground. How are conditions? If your locomotive has to work hard to keep schedule under normal conditions, if you get some bad conditions (high headwinds, etc) then it'll tend to run late.
Speed costs money, and the U.S. railroad industry has long since found it more profitable to make good money hauling relatively time-insensitive cargo at modest speeds. Coal doesn't care if it takes an extra day to get to where its going. Lot of modern diesels are only geared for ~65 MPH maximum.
