Re: Do 6 axles outpull 4 axles?

If the SD-7 is heavier it will pull more, assuming the same type of wheel material. Number of driving wheels in this case is unimportant unless there is limited sideplay, which means you can get some extra traction from flange friction.

Much less than you would expect! The rail top generally has a curved/domed cross section while the wheel treads have a cone shape. In theory, the point of contact would be a point with zero dimensions. The rail head and wheel tread deform fractionally until the weight is spread sufficiently for support. The problem I was attempting to describe is that that deformation is sustainable so long as the elastic limits of the steel are not exceeded, beyond that point the deformation becomes permanent. The steel can be made harder to push the limit further up the scale but then it is more subject to fracture and becomes more expensive because it's purity has to be increased. Alternatively, you can just accept the rapid wear on the rails/wheels, as has been done in the USa. Generally, wheels are made harder than rails so that the rails take most of the wear. In that situation, when you harden the (new) rails you need a corresponding increase in hardness of wheels which then increases the likelihood of tyre fractures. On slow running US trains this increase is obviously tolerated, while in Europe on high speed trains, the axle loadings are reducing from 25/22 tonnes to around 17 tonnes.

No! (assuming you are using NMRA profile wheels) The flange and tread of a railway wheel can never touch the railhead at the same moment because the fillet between them is of larger radius than the top corner of the rail head.

If you're using wheels without a fillet (eg Lima, Kleinbahn etc) then the flange and tyre can both bear at once, but in that case the bearing points are at different radii so the proportional difference between those two radii in torque is lost. (example HO tread radius 6mm/flange rubbing point 6.5mm = 92% or 8% torque lost in friction)

In model form this will tend to clean the outer rail, which will improve current collection and thereby possibly improve pulling power.

Regards, Greg.P. Takaka, NZ.

Over the years rail head size has increased. One reason is to take larger loads by increasing contact area. The curved top of the rail is over time is worn down, just as the wheel wears, again increasing contact area. Some railways machine wheels to a worn wheel profile to get this advantage, and thus decrease overall wear. Now the reason why high speed trains have a lighter axle load compared to slow trains using the same rail section is simple. Higher speeds generate higher forces in the suspension system which is transmitted into the wheel and rail. The dynamic loads are similar for each case. Another reason for light axle loads at high speed is the un sprung mass needs to be kept as small as possible at high speeds in order to keep the inertia of the wheel and axle down, otherwise the wheel might lift off the rail. You probably already know this, you are right in that there are trade offs between steel selection and loading, and the need to stay within the elastic limit of the materials. I am sure axle loads and speeds will continue to increase when economics and technology allow.

Sounds good in theory but wether the flange or the tread are both in contact or not does not impact overall on the average traction of the model. It is a function of the weight of the model acting through the force component normal to the various points of contact with the rail., and the coefficient of friction. Model trains have far in excess torque supplied by the motor so the above loss of torque is unrelated to the overall friction force between the model and the rail. Wheel slip will occur no matter what flange profile is used, simply because the distance travelled on the outer rail is greater than the inner rail.