Russo Loco Wrote:
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> Philip et al -
>
> I vaguely remember an old rule of thumb that the
> top speed in miles per hour is approximately 110%
> of the driver diameter in inches, so a K-36 should
> max out at 44 × 1.1 = 48.4 mph (about 50),
> and #4449 or #844, etc., should max out at 80
> × 1.1 = 88 mph (about 90).
That's probably a decent "ballpark figure" for the sustained speeds of a late model, well-balanced locomotive designed for the task of high-speed cruising. UP 844 or SP 4449 could cruise at 90 MPH for hours on end, with burst speeds exceeding 100MPH certainly possible. Doesn't apply so much to freight locomotives. I'm not sure that D&RGW K-36 could even reach 50 MPH without either shaking itself apart or leaving the track, and if it could it certainly wasn't sustaining it for any length of time! That's a fair distinction to make, because absolute maximum speed, practical maximum, and sustainable speed over long distance are all different things. The N&W J 4-8-4 was demonstrated to have an actual top speed exceeding 110 MPH if the user didn't mind breaking the locomotive, while its practical maximum was more like 90 for moderate bursts, and its long-distance point-to-point sustained speeds, less still. Again: The practical maximum and the physical maximum are not always the same thing with respect to steam locomotives, particularly for modern high-power designs. Railroads didn't want to wear out their locomotives or damage their track any more than you want to ruin your family car's powertrain and tires by constantly running it as hard as you possibly can. Speed is expensive.
Point of the above being: The generic term "Top speed" of a locomotive can have quite a few different meanings, depending on context, and even ignoring outside (but still critical) factors like the quality of track, safety requirements, and whatnot. You might get three or four different answers for the same locomotive--and they can all be correct.
Steam engines hate speed. The engine drives the locomotive. It is possible to make a locomotive run fast, but the engine itself must be kept fairly slow. Locomotive steam engines are happy enough running at 150 or 200 RPM, and even the best from the late steam era will not tolerate 500 RPM without eventually failing. There is no hard rule because technology changed so much over the years. Maximum theoretical locomotive speed is always governed in part by maximum engine speed (available power permitting), and that can vary wildly from one design to the next. For many years ~350 RPM or so was regarded as the practical maximum for regular use, faster speeds only being achieved in short bursts. That dated back to the days of slide valves, plain brass bearings, iron rods, and tallow-based lubrication. Give a locomotive piston valves with modern lubricants and light roller-bearing steel rods/running gear with good balancing and you can get about 100RPM more out of it with acceptable reliability. Even then, failures will be reduced by running slower. Can't get around it--reciprocating steam engines hate speed.
In addition to engine RPM, lubrication and material limits (and to a degree, valve timing) tended to want to limit practical piston speed, too. As lubrication improved, allowable piston speeds continually increased. During the 1870's, 1000 feet per minute was a fairly fast piston speed, and 1600 or so was about the practical maximum. The V&T "INYO" was supposedly good for up to about 60 miles per hour, at which speed it would have been running at a little over 1400 feet per minute piston speed and slightly over 350 RPM engine speed--both near or at what would've been regarded as effective maximums for the 1870's. I think it's pretty neat how all the different aspects of the design end up in good synergy like that. It was a good locomotive that led a very long and useful service life. Move on some decades later. Take UP 844. At a little over 100 MPH, which the locomotive was certainly capable of, the engine would be running at about 450 RPM, with a piston speed in the range of 2400 feet per minute. The technology had improved. That was about the best it got.
If you want to run fast, use an electric. Steam or diesel? For speed, it hardly matters. In either case they're severely limited by the need of the locomotive to lug around its own power source (either a boiler or a prime mover). That becomes a serious problem for high speed given the massive amounts of power required for fast running. You can only carry around so much boiler or diesel engine on a railroad locomotive. Hence diesels don't really run any faster than steam did (they just do it more cheaply/reliably), and even the fastest of them aren't THAT much faster than the fastest steamers. Electrics make either look pathetic by virtue of their being able to use an off-site, and hence unlimited in size, power plant. It's not cheap and it's not exactly efficient, but if you want to run a railroad fast, that's the only realistic way to do so at the present.
Track gauge hardly matters except in terms of stability (particularly on curves) and for the need to mount electric traction motors between the axles. Both of those factors favor wider rail spacing. Brunel was right; he was simply 150 years ahead of his time.
