ChandraS wrote:RPF is not wrong. The gage between the rails is slightly increased on curves depending on radius of the curve and the speed to be sustained. This is usually a few mm only. RPF's argument is the basis of this.
Ah..! Thanks ChandraS. BRF Rocks!. There is no subsitute to actually having people who are hand on and do it. No amount of googling would give out information like that
gauge between the rails is changed on curves! So that
change is how you get the "toe in" forces in railways and get the train to start turning.. the gauge change , which gets the train started in the turn , along with the "cant" or "tilt" (the well know tan thetat yada yada in high school) is what takes the train round the curve!. From you I learnt that 1) That tracks are not perpendicular but have an inclination (I thought that the wheel shape was "concave "and the rail shape was "convex" so that they have a good line contact at the bearing surfaces. I would have never thought that the engineering practice would be to
tilt the track, while I think it would be far easier to maufacture a track with a surface shape that will fit the conical side of the wheel !
How you get the toe in forces in automobiles is that the suspension
tilts the wheel. When you turn the wheel left, the wheels in addition to turning left, also tilt ever so slightly to the left, getting the "toe in" forces which starts the turn. If not it will require a huge amount of force (you will have to work against friction) to turn. Same is the reason why all wheel independent suspension cars will have better handling, becuase in that case the rear wheels too can "toe in" in addition to the front wheels.
ChandaS, could you take some time to think over and rack your brains on this to recollect old thoughts?. I would think that to get the train to turn to the left, what you want to do is to get the train to ride in the "narrower" cone portion of the left wheel and make it ride on the "wider" cone portion of the right wheel, so that it turns left. For that, you would actually start by
decreasing the gauge initally from the right side to start the turn, while simeltaneously getting the "bank" going to get the centrifugal forces in play to keep it turning.
Von Ryan's Express is a pretty good movie but can hardly qualify to be an argument against RPF
Oh. Von Ryan's express served pretty well. It is only in old movies like that yous see the trains slide and sparks flying under emergency stop . That scene (if it was that movie) was well filmed. Obviously not something you get to see in everyday life.
However, I think the "victorian 19th century" / Feynman explanation based on kinematics is flawed, because it assumes point contact /kinematics as the reason why the train stays on "track" and is self steering. Like you explained,
steering a train is carefully engineered , it doesnt do that by itself
Think of a real world case, if you think that Von Ryan express emergency braking/sliding train is all Hollywood special effects. If the "Feynman" explanation is correct, what will happen if some guy engineered a train where the left and right wheels are not connected by a solid shaft and hence able to rotate at independent speeds, like the rear wheels of a front wheel drive car or the front wheels of a rear wheel drive car ?. The "Feynman explanation" would say that the wagon wont be able to stay on track in a straight line!
Will you call that designer a Paki ?
If so , how do you explain this bogie design ?
Check out this link
For full size.
So, by the "classic" kinematic explanation, the tracks and wheel shapes should be different for this design and such a train cannot run on the same tracks as a "conventional" train, right ?
If so, check out this
Amtrak Train which is from the company
Talgo, which uses the bogie design as above!. It runs as the same track as the normal conventional train!. Now when the bottom is knocked out of the "Kinematic explanation" ie that fixed shaft connecting the wheels is what gives straigth line stability, you know that it is lot more complex than that!. .. Engineers intuition right I guess!

.
But seriously, BRF rocks. Who would have thought that someone like me, the closest I have come to rail tracks and wheels is at railway crossings and at stations would get so much insight into how the trains actually stay on track ,all in the space of two "cigarette breaks" and discussion with a bunch of very smart folks like you and Amber G and learn non intuitive stuff like tracks are actually "tilted" and that gauges "change", and then the opportunity to dig deeper and examine stuff critically and google/reasrch for stuff that can prove the "conventional wisdom" as inadequate. A real mental workout, makes you feel like you are 16 again and a bright spark with great inquisitiveness!
So .. Is RPF ("classical Kinamatic explanation") wrong ? Are the Talgo bogie and train designers Pakis?
Hint
Feynman / Classical Kinematic explantion is right of course and the Tolgo Train designers are Spanish and NOT Pakis! . But how can both be right ?.
Becuase there are TWO questions 1) How does the train "auto guide" ie steer itself through corners setc and the 2) How is the train laterally stable, ie if there is a disturbance in the transverse direction, it comes back to it's stable position, even if the wheels are not rolling like in Von Ryan's express . The answers to both is the shape of the wheels. Yes, if the wheels are independent, the train will not "auto guide" , which in all fairness is what RPF talked about in the video. The answer to the 2nd question is what keeps the train laterally stable, and that is what I answered. The confusion comes in the way the question was framed as " What keeps the train on its tracks"? by Amber G , ie. how does the train know where the center of the track and therefore it can follow curves. That really is two questions with the same aswer. But are the Talgo guys Pakis?.. coz, if Feynman is right, their train while being laterally stable, will not be able to go around curves! No they are Spanish, because they provided active steering for their train!.
ChandraS, your answers were spot on. Thanks,you helped me a great deal in undertanding that there were 2 problems actually. I threw the Talgo train at you hoping that you were not familiar with it and could throw your core understanding in doubt! Did you go to that Railway Engg school that is somewhere in the Eastern part of the country .. In any case, the Railways obviously do a very good job in training their folks. A really nice thing to know