As a kid, the first thing that came to my mind on hearing the word "train" is "thadakh-thadakh" sound. Have you ever wondered where that "thadakh-thadakh" comes from? Since ten years of age, this question has been bothering me and not until few months ago did I strike upon the answer! I am still not 100% sure whether I have found the right answer, but I am 99.99...9% sure.
When I went about asking this question to the elders back then, one of them answered that the sound came from the gravel stones that lay over the tracks. "But how?", I exclaimed helplessly, only to avail his silence (actually he said the stones dance while train goes above it, it dances to a rhythm just like how heroes dance in the movies to a rhythm. What on earth was the bastard thinking of me?). Another wise person said the sound came from the locomotive engine. I asked back how come the sound then is so clear and audible even in compartments that are so far away from the locomotive? He then panicked a bit and replied, "Then the sound must come from compartments!". I sensed the feigning philosopher in him and decided not to believe anything he says.
Then I thought about this myself, after all the sound HAD TO come from the compartment. Because even when, as a kid, I used to have fun in strolling from one compartment to another (keeping one leg in one compartment and another leg in another compartment simultaneously at intersection of coaches is something almost all middle-class Indian kid would've done secretly in the excuse of going to toilet) and I've never observed the absence of thadakh-thadakh in any compartment. In AC compartments, I noticed the volume of thadakh-thadakh sound reduced considerably, so it was apparent the sound did not originate from inside (cabin) the compartment but from outside (since all windows are sealed in AC coaches, it was a reasonable argument).
But I could only get this far. Time then flew, this particular nagging question fell into the oblivious section of my mind. Then in 11th grade, this rekindled. Now I was wary of Newton's laws. I knew rotational mechanics and also the mechanics behind rolling. But none of them provided any answer as to why a smooth wheel that is simply rolling (even with friction) should give out periodical sounds (unless the wheel had any slight bump, of course). It intrigued me a lot, the answer was still out of my reach.
One day (while travelling in train, of course), I noticed the thadakh-thadakh got DAMN MESSY when the compartment switched tracks. It was something like thad-thadaa-thadaa-tha-tad-thad-thadaa-thadaa-tha-tad-thadaa... followed by the usual thadakh-thadakh...thadakh-thadakh...
This is a vital piece of information. This says track too has a role to play and not just the compartment. So what could it be? What is the root behind this sound?
The answer lies in the fact that metals expand when heated. :-) Okay, that's a bit vague... let me explicate.
When the engineers constructed the tracks, they could not do it in one go. They made segments of tracks and then joined them. But did they weld two joints together? No, not always. They DELIBERATELY left a few millimeters of gap between one track and another. Like this:
This gap is crucial. The reason behind this is that during summer and spring times, the scorching heat from the sun will heat up the track metal. Metals have the property of expanding when heated (to certain extent). The extent (del L by L) to which they expand depends on the temperature to which it is raised (del T) and on the property of metal (alpha, which is a constant).
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But hey, what happens if you don't leave any gap at all? Well, then the metals expands side-wards and upwards and then the whole track will get crazily distorted into something like this:
Okay, so what does this gap have anything to do our thadakh-thadakhs?
The answer is that when the wheel of compartment crosses over this gap, the wheel BUMPS and this bump will produce a BIG jerk in the suspension of the wheel as it lowers a bit and rises when it passes the gap.
Why should it lower and rise? It is as simple as you bumping into a small pit on the road where your car wheel almost falls into the pit & then bumps and passes over the pit.
See this rail gap:
When the wheel bumps (pardon my pathetic MS paint skills):
But behold, why are there TWO thadakhs? There are two thadakhs because in all the coaches, the wheels always come in pairs. First thadakh for the first wheel and the next immediate thadakh for the wheel that is right behind the first wheel.
The reason why that big a sound is produced while encountering just millimeters of gap is because the coach is so damn heavy (about 40 tonnes) that even minute bumps tend to produce enormous normal reaction force. You can also correlate this solution with points that were noted earlier. For instance, the thadakhs go all over the place when it switches tracks because there are so many bumps for the wheel (and its suspension system) to deal with while tracks diverge and converge. Also, you might have noticed the pace of thadakh-thadakh reduces as the train speed reduces. This is because the wheel covers shorter distance in longer time and hence encounters the gap on the rails at a slower rate. Thaaaadakkkhhh-----------thaaaaadakkkhhhhhhh....
I suddenly realized all this while crossing the rail-cross as I saw the gap between rails. I then waited for the train to come for verifying my hypothesis and... success! Goosebumps ran all over my body! One of my childhood inquisitions can now rest in peace. Time to move on to next childhood inquisition! ;-)
When I went about asking this question to the elders back then, one of them answered that the sound came from the gravel stones that lay over the tracks. "But how?", I exclaimed helplessly, only to avail his silence (actually he said the stones dance while train goes above it, it dances to a rhythm just like how heroes dance in the movies to a rhythm. What on earth was the bastard thinking of me?). Another wise person said the sound came from the locomotive engine. I asked back how come the sound then is so clear and audible even in compartments that are so far away from the locomotive? He then panicked a bit and replied, "Then the sound must come from compartments!". I sensed the feigning philosopher in him and decided not to believe anything he says.
Then I thought about this myself, after all the sound HAD TO come from the compartment. Because even when, as a kid, I used to have fun in strolling from one compartment to another (keeping one leg in one compartment and another leg in another compartment simultaneously at intersection of coaches is something almost all middle-class Indian kid would've done secretly in the excuse of going to toilet) and I've never observed the absence of thadakh-thadakh in any compartment. In AC compartments, I noticed the volume of thadakh-thadakh sound reduced considerably, so it was apparent the sound did not originate from inside (cabin) the compartment but from outside (since all windows are sealed in AC coaches, it was a reasonable argument).
But I could only get this far. Time then flew, this particular nagging question fell into the oblivious section of my mind. Then in 11th grade, this rekindled. Now I was wary of Newton's laws. I knew rotational mechanics and also the mechanics behind rolling. But none of them provided any answer as to why a smooth wheel that is simply rolling (even with friction) should give out periodical sounds (unless the wheel had any slight bump, of course). It intrigued me a lot, the answer was still out of my reach.
One day (while travelling in train, of course), I noticed the thadakh-thadakh got DAMN MESSY when the compartment switched tracks. It was something like thad-thadaa-thadaa-tha-tad-thad-thadaa-thadaa-tha-tad-thadaa... followed by the usual thadakh-thadakh...thadakh-thadakh...
This is a vital piece of information. This says track too has a role to play and not just the compartment. So what could it be? What is the root behind this sound?
The answer lies in the fact that metals expand when heated. :-) Okay, that's a bit vague... let me explicate.
When the engineers constructed the tracks, they could not do it in one go. They made segments of tracks and then joined them. But did they weld two joints together? No, not always. They DELIBERATELY left a few millimeters of gap between one track and another. Like this:
This gap is crucial. The reason behind this is that during summer and spring times, the scorching heat from the sun will heat up the track metal. Metals have the property of expanding when heated (to certain extent). The extent (del L by L) to which they expand depends on the temperature to which it is raised (del T) and on the property of metal (alpha, which is a constant).
But hey, what happens if you don't leave any gap at all? Well, then the metals expands side-wards and upwards and then the whole track will get crazily distorted into something like this:
Okay, so what does this gap have anything to do our thadakh-thadakhs?
The answer is that when the wheel of compartment crosses over this gap, the wheel BUMPS and this bump will produce a BIG jerk in the suspension of the wheel as it lowers a bit and rises when it passes the gap.
Why should it lower and rise? It is as simple as you bumping into a small pit on the road where your car wheel almost falls into the pit & then bumps and passes over the pit.
See this rail gap:
When the wheel bumps (pardon my pathetic MS paint skills):
But behold, why are there TWO thadakhs? There are two thadakhs because in all the coaches, the wheels always come in pairs. First thadakh for the first wheel and the next immediate thadakh for the wheel that is right behind the first wheel.
The reason why that big a sound is produced while encountering just millimeters of gap is because the coach is so damn heavy (about 40 tonnes) that even minute bumps tend to produce enormous normal reaction force. You can also correlate this solution with points that were noted earlier. For instance, the thadakhs go all over the place when it switches tracks because there are so many bumps for the wheel (and its suspension system) to deal with while tracks diverge and converge. Also, you might have noticed the pace of thadakh-thadakh reduces as the train speed reduces. This is because the wheel covers shorter distance in longer time and hence encounters the gap on the rails at a slower rate. Thaaaadakkkhhh-----------thaaaaadakkkhhhhhhh....
I suddenly realized all this while crossing the rail-cross as I saw the gap between rails. I then waited for the train to come for verifying my hypothesis and... success! Goosebumps ran all over my body! One of my childhood inquisitions can now rest in peace. Time to move on to next childhood inquisition! ;-)
