Comparing SD70Ace to Stephenson's Locomotion #1 - Trains Magazine
The confusing thing about looking at tractive effort is that on it's own it doesn't tell us anything other than potential. The other thing to consider is adhesion or actual traction. Tractive effort is actually torque as measured at the rail. You can have all the TE in the world but it does no good if you don't have the adhesion (traction) and are just spinning your wheels. Since torque can also be thought of as leverage, we need to look at how this affects potential adhesion. As wheel diameter goes up, TE goes down (all things being equal) at lower speeds. Think of that larger wheel compared to a smaller wheel as a gear in a car. A larger wheel is a higher gear and a smaller wheel is a lower gear. A steam engine with it's larger wheels has less starting TE per the same amount of drawbar horsepower. It sounds confusing but is easy to explain with math if you want me to get that complex with it.
Let's again compare wheel sizes to torque or leverage. We know that a car is going to spin it's wheels best in low gear. In higher gears it doesn't do it as easily if at all. Same thing with steam engines vs diesels and how it relates to adhesion. Our steam engine may have good horsepower but is perpetually stuck in "high gear" with it's larger wheels. That equals less starting TE which will mean it will have a harder time getting a train moving from a stop but it will also mean that it will have a better TE to adhesion ratio. It can get most of it's power down without spinning the wheels. It's not to say that it can't though as steam engines definitely did spin their wheels.
If you get a diesel engine moving with it's smaller diameter wheels, as long as it can maintain adhesion, it can accelerate faster. Again think of this as starting a car out in high gear vs low gear. Now pretend you are doing it on ice. Both can probably break the wheels loose but which one has the potential to accelerate you faster?
However the advantage to the smaller wheels will quickly go away as speed increases. How efficient is your car on the freeway in a lower gear? You don't need as much TE to keep moving the faster you go but at some point your TE will fall below your available adhesion limit which means no matter what you do, you can't go any faster as all you can do is spin your wheels.
It gets confusing and is a bit hard to explain clearly without only bringing up more questions. Needless to say it is an interesting topic that can definitely be explained in far more depth. I am greatly simplifying things and leaving some things out such as available power (or steam) at various speeds.
