Re: Kinky 3-rail vs staight 4-rail turntables . . .

bcp Wrote:
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> Three-rail turntables don't work if they are that
> short.  But do if they are realistically longer.


True enough, Bruce -

For clarity, part 2 of my above drawing exaggerated the offset of the rails, but your drawing ignored the kink when a three-rail TT is used to turn an engine all the way around, so here's an exaggerated drawing of the kink that would result with a really short turntable — assuming that we're talking about dual 4' 8½" and 3' gauge:


Note that the end of the turntable would have to be moved an extra 20.5 inches to line up the common rails with the center rails.  With a 50-foot TT, that would create an angle of 20.5" / 300" = 0.0683, the arctan of which is 3.91 degrees.  A 3.91° kink is probably not too much different from the kink when an engine passes over the points of a switch, but it's just one more factor that might contribute to a derailment (not to mention what happens if the hostler puts my namesake Johnson Bar into reverse . . . )   And, as you pointed out above, the longer the turntable the less kinky the get-together;  a 100-foot three-rail turntable would have a misalignment of only about two degrees in the case illustrated here.


Hank Wrote:
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> That strikes me as a more complicated and failure
> prone solution. It seems to me you'd have balance
> problems, need two locks at each TT lead, & that
> there would be a slight angle between the Turn-
> table bridge and the approach tracks when in the
> offset position shown.  Much easier, and safer, to
> just have draws on the approach track(s).  Might
> not even be much more expensive. . . .
>
> Question (for those old enough to know):  At dual-
> gauge roundhouses (Alamosa, Salida in later years)
> did the D&RGW run 4 rails all the way into each
> stall or were there draws on the RH tracks too?
> And what did the C&S do at Denver?


Might need three locks or maybe even four, Hank -

Depending on which end of the Table is transitioning to the third rail setup, thus adding to the possibility of confusion and increasing the likelihood of accidents.  Cutting down on yard crew decision making may even be the primary motivation for having four rails instead of three — the old K.I.S.S. principle in action!

IIRC, the tracks into the roundhouse were either 36" gauge in the old section(s) or 56.5" gauge in the new section(s) added later.  SFAIK there were few, if any, stalls with either three or four rails to accommodate both narrow- and standard-gauge locomotives.  See John West's December, 1961 shot of #480 (& #488?)
and September, 1964 shot of #498 in the Alamosa roundhouse at [chasingtrains.smugmug.com] and [chasingtrains.smugmug.com] . . .

- Sincerely, Willie  (Wm. Claude Johnson-Barr III, Esq.)  
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Edited 7 time(s). Last edit at 07/01/2022 06:35PM by Johnson Barr.