. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . le. The virtual profile shows theactual state of the case and only points out the necessity, if anunexpected requirement for a full stoppage of trains at a criticalpoint has developed, of changing the location (if a station), orof changing the grade by regrading or by using an overheadcrossing. 518. Undulatory grades. Advantages. Money can generallybe saved by adopting an actual profile which is not strictlyimiform—the matter of compensation for curvature being here 574 KAILROA
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . le. The virtual profile shows theactual state of the case and only points out the necessity, if anunexpected requirement for a full stoppage of trains at a criticalpoint has developed, of changing the location (if a station), orof changing the grade by regrading or by using an overheadcrossing. 518. Undulatory grades. Advantages. Money can generallybe saved by adopting an actual profile which is not strictlyimiform—the matter of compensation for curvature being here 574 KAILROAD CONSTRUCTION. § 518. ignored. Its effect on the operation of trains is harmless pro-vided the sag or hump is not too great. In Fig. 215 the undu-latory grade may actually be operated as a uniform grade sag at C must be considered as a sag, even though BC is actu-ally an up grade. But the engine is supposed to be workinghard enough to carry a train at uniform velocity up a grade it gains in velocity from B to C, and from C to D losesan equal amount. It may even be proven that the time re-. Fig. 215. quired to pass the sag will be slightly less than the time requiredto run the uniform grade. Disadvantages. The hump at F is dangerous in that, if thevelocity at E is not equal to that corresponding to the extravelocity-head ordinate at F, the train will be stalled beforereaching F. In practice there should be considerable train should have a velocity of at least M (see § 455)in passing any summit. An extra heavy head wind, slipperyrails, etc., may use up any smaller margin and stall the the grade AG is a ruling grade, then no bump should be allowedunder any circums tances. For the heaviest trains are supposedto be so made up that the engine will just haul them up theruling grades—of course with some margin for safety. Anyincrease of this grade, however short, would probably stall thetrain. Safe limits. Since over of all freight c
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