>Why do trains derail and how can you prevent this from happening?
This is simply a question of "mechanics", that is to say the mechanics (Newtonian?) which is a branch of applied mathematics rather than relating to machinery. For most people it is not a simple question to answer and I won't attempt to do the maths for fear of embarassing myself and boring people to death. However, by examining the principles of Mechanics we should be able to deduce a set axioms that will allow us to analyse why our trains come off the rails.
So here's a few axioms to start us off:-
1) A body in motion will continue in a straight line unless it encounters a force which acts on it to change its direction. EG. A pool/snooker/billiard ball will travel straight on until it hits another ball or a cushion.
2) All forces acting on a body can be resolved into 3 dimensions - vertical, horizontal (left / right), horizontal (forward/back). Once resolved into these 3 dimensions, an increase in force in one dimension has no affect in the other two. For example if our pool ball rolls off the table, it will drop straight down without deviating left or right from its original course.
3) If you pull on two ends of a piece of string it will form a straight line between the two points where the force is applied. Now think about train couplings.
So how can we apply this to derailments?
For a derailment to take place, the flange of one of more wheels must rise above the level of top of the rail /and/ there must also be a sideways force to push the wheel outside the confines of the track.
So having established that we need forces in two of our three dimensions for a derailment to take place, where might these forces originate?
From the first axiom, we can deduce that forces will arise if the wheels hit something in their path. This could be:
entering a curve
hitting a frog or check rail in a point or crossing
riding over a joint in the track
In each of these cases, there will be both a sideways and vertical component in the force. In the case of entering a curve, the vertical force arises from the profile (shape) of the wheels which have a slight camber to them. On entering the curve, the wheel wants to travel straight on, so the curved wheel profile will start to raise the wheel. The force acting against this upward vertical movement is the weight of the vehicle.
The case for force resolution into sideways and vertical dimensions for points is much clearer. The jolt of a check rail or frog will produce a sideways correction and almost certainly an upward movement as well.
Another force acting against perfect running was mentioned by Columbo in a recent thread on couplings. If the link between vehicles is not made through the centre line of those vehicles, a turning force will be applied the them. (cf axiom 3 above) The "neutral" position for that vehicle coupling (ie no more sideways component) occurs when the line of action of the force through the coupling is parallel to the track. Since the line of action of the coupling force is not through the vehicles' centre line, this neutral position cannot be achieved until one of both vehicles is off the rails - ie this is a derailment waiting to happen; there is plenty of sideways force ready and waiting for sufficient upward force to attain equilibrium.
So how can we prevent derailments?
From the above it is apparent that by reducing all opportunities for disturbing forces to be created, derailments can be also be reduced. To do that here are some guidelines. I am sure there are many more:-
1) Be fastidious when laying track. This is more important for flexible track than set track. Any uneveness in rail height creates a location for vertical movement.
2) Don't place points too close to the start or end of a curved section. In a small space it is tempting to squeeze a quart into a pint pot by ending a curve and placing a point but in my experience it often ends in tears.
3) If a particular vehicle has problems with a point or crossing, check the wheel set is within specification for the standard of rail you are using - This problem has been mentioned quite a few times in threads on this forum
4) Don't allow large gaps between rail ends. If the gap is large enough, the wheels will drop in and then bounce up again on the way out. Any sideways movement through the coupling and you have a disaster. (I have this particular problem on a code 100 section of my layout at present).
5) Ensure that all vehicles run freely and are reasonably matched for weight. Light vehicles at the front of a long train with heavy vehicles at the rear can easily result in a derailment round a bend.
6) Ensure that all couplings mate so that the force under tension is distributed so that it is acting along the centre line of the vehicle when it is on a straight section of track.
The NMRA has guidelines for things like wheel profiles, vehicle weights and so on and I am sure they are the result of a mathematical analysis.
For what it's worth, I can run a Hornby A4 drawing 6 Gresleys at full speed over code 75 pointwork, forwards or reverse.
At the end of the day it is all governed by the laws of physics. There's no magic or black art to it.
David
This is simply a question of "mechanics", that is to say the mechanics (Newtonian?) which is a branch of applied mathematics rather than relating to machinery. For most people it is not a simple question to answer and I won't attempt to do the maths for fear of embarassing myself and boring people to death. However, by examining the principles of Mechanics we should be able to deduce a set axioms that will allow us to analyse why our trains come off the rails.
So here's a few axioms to start us off:-
1) A body in motion will continue in a straight line unless it encounters a force which acts on it to change its direction. EG. A pool/snooker/billiard ball will travel straight on until it hits another ball or a cushion.
2) All forces acting on a body can be resolved into 3 dimensions - vertical, horizontal (left / right), horizontal (forward/back). Once resolved into these 3 dimensions, an increase in force in one dimension has no affect in the other two. For example if our pool ball rolls off the table, it will drop straight down without deviating left or right from its original course.
3) If you pull on two ends of a piece of string it will form a straight line between the two points where the force is applied. Now think about train couplings.
So how can we apply this to derailments?
For a derailment to take place, the flange of one of more wheels must rise above the level of top of the rail /and/ there must also be a sideways force to push the wheel outside the confines of the track.
So having established that we need forces in two of our three dimensions for a derailment to take place, where might these forces originate?
From the first axiom, we can deduce that forces will arise if the wheels hit something in their path. This could be:
entering a curve
hitting a frog or check rail in a point or crossing
riding over a joint in the track
In each of these cases, there will be both a sideways and vertical component in the force. In the case of entering a curve, the vertical force arises from the profile (shape) of the wheels which have a slight camber to them. On entering the curve, the wheel wants to travel straight on, so the curved wheel profile will start to raise the wheel. The force acting against this upward vertical movement is the weight of the vehicle.
The case for force resolution into sideways and vertical dimensions for points is much clearer. The jolt of a check rail or frog will produce a sideways correction and almost certainly an upward movement as well.
Another force acting against perfect running was mentioned by Columbo in a recent thread on couplings. If the link between vehicles is not made through the centre line of those vehicles, a turning force will be applied the them. (cf axiom 3 above) The "neutral" position for that vehicle coupling (ie no more sideways component) occurs when the line of action of the force through the coupling is parallel to the track. Since the line of action of the coupling force is not through the vehicles' centre line, this neutral position cannot be achieved until one of both vehicles is off the rails - ie this is a derailment waiting to happen; there is plenty of sideways force ready and waiting for sufficient upward force to attain equilibrium.
So how can we prevent derailments?
From the above it is apparent that by reducing all opportunities for disturbing forces to be created, derailments can be also be reduced. To do that here are some guidelines. I am sure there are many more:-
1) Be fastidious when laying track. This is more important for flexible track than set track. Any uneveness in rail height creates a location for vertical movement.
2) Don't place points too close to the start or end of a curved section. In a small space it is tempting to squeeze a quart into a pint pot by ending a curve and placing a point but in my experience it often ends in tears.
3) If a particular vehicle has problems with a point or crossing, check the wheel set is within specification for the standard of rail you are using - This problem has been mentioned quite a few times in threads on this forum
4) Don't allow large gaps between rail ends. If the gap is large enough, the wheels will drop in and then bounce up again on the way out. Any sideways movement through the coupling and you have a disaster. (I have this particular problem on a code 100 section of my layout at present).
5) Ensure that all vehicles run freely and are reasonably matched for weight. Light vehicles at the front of a long train with heavy vehicles at the rear can easily result in a derailment round a bend.
6) Ensure that all couplings mate so that the force under tension is distributed so that it is acting along the centre line of the vehicle when it is on a straight section of track.
The NMRA has guidelines for things like wheel profiles, vehicle weights and so on and I am sure they are the result of a mathematical analysis.
For what it's worth, I can run a Hornby A4 drawing 6 Gresleys at full speed over code 75 pointwork, forwards or reverse.
At the end of the day it is all governed by the laws of physics. There's no magic or black art to it.
David
