The theoretical pulling power of a steam locotive depends on several factors, the main three of them being:
1. Cylinder horsepower: This is the maximum power developed by the cylinders at full boiler pressure.
2. Boiler horsepower: At a certain speed, the cylinders will use steam faster than the boiler can produce it. This is where an engine must be "hooked up" (bring the johnson bar towards center). This sets the valve gear to limit the cutoff and the amount of steam that is admitted to the cylinders and reduces the cylinder horsepower. Several factors determine this, including driver diameter/piston speed and heating surface area. A 2-8-2 will outpull a 2-8-0 of the same te, cylinders and drivers at higher speeds because the 2-8-2 can carry a larger boiler and firebox.
3. Factor of adhesion: A factor of 4 means that the maximum tractive effort is 25% of the weight on drivers. For steam and early diesels, this is the most common figure used. Too little weight on the drivers and the engine will be slippery, too heavy a weight is a waste.
Changing an engine from a tank to a tender will change the factor of adhesion. An tank engine full of water will be carrying a substantial extra amount of weight on the drivers, allowing the engine to pull harder without slipping. With a near empty tank, the engine must still have a factor of near 4 or it will slip.
Does removing the tanks from an engine make it more slippery? Yes, but if done right it should not be any different than any other tender engine. SVRY added weight in the form of old rails on the running boards of the 250 and 251 (notice how thick the running boards are in photos). This was probably to replace the weight of the tank itself and to maintain 4 times the te as weight on drivers. Most articulateds were claimed to be slippery when they were changed from tank to tender, but in my opinion this might have also had to do with the probable failure to properly rebalance the weight and springing between the front and rear engines.
Diesels have almost unlimited pulling potential for a very short time when starting, the starting tractive effort is limited by adhesion determined by the weight on drivers (25% or factor of 4), though it can vary from 20% (factor of 5) on poor rail up to 33% (factor of 3) on sanded good rail. The newer wheelslip systems and AC drive now have them pulling 40%+. That is why most smaller infernal combustion locomotives are often referred to by their weight, a quick reference to their maximum pulling power at low speeds.
