Re: K36/K37

The running gear of a K37 is essentially identical to a K36 from the pilot to behind the rear driver. The rear frame extention under the firebox and the trailing truck is completely different. There are some minor differences in the castings of the cylinders and valve gear link hanger, but nothing major. As the firebox bottom of the K37 is different shaped and longer than a K36, a different rear frame is used. Where BLW used a nice cast, curved member on the K36, the DRGW opted for a big square piece with angle bends to fit (these rear frame members were prone to breakage, several K37's show welded repairs and patching - 497 has both).
The trailing truck on the K36 is a "Hodges" type with the wieght born on leaf springs and swing links. When the truck swings laterally, the links increase the load on the springs, which acts as a centering device. It is a very flexible truck, but has little centering forces.
The K37 trailer truck uses leaf springs to push down on a flat pad attached to the locomotive frame. This pad mates with a pad on the truck frame, applying downward pressure to support the rear of the locomotive. Centering is accomplshed by a pair of coil springs attached to the rear of the truck and to the main frame. The result is a truck with greater centering forces. BTW, this is the same trailer truck used on K28's
Some think the K37 truck is too stiff and makes the truck prone to derailment. We never had any trouble. The K37 truck does anchor the rear of the engine down and make for a better ride.
Also, because of the longer firebox, the trailer truck axle is a foot further back, compared to a K36.
K37's have a larger boiler (about 6" bigger I think). In order to fit a larger diameter, the saddle under the smokebox is machined to a larger radius.
While the K37 boiler is larger diameter and has a larger firebox, it has a shorter barrel. To make it fit the frame, the smokebox is quite a bit longer. If you look at a photo of a K37 with a light colored smokebox, you will notice there is a considerable distance from the stack to the back of the smokebox.
Another difference is shorter #4 driver springs on the K37 (to clear the firebox).
Many running gear parts are interchangable. Rods, valve gear parts, pistons, valves, drivers, driving boxes, springs, shoes, wedges, pilot truck etc were built to the same dimensions (which is to say they are probably NOT the same now).
Back in the mid 80's, we put 495's drivers under the 484. The only difference we found was BLW used one bolt on the eccentric crank to hold it on the crankpin, where the DRGW decided it needed two. I don't think the DRGW ever swapped drivers between classes like we did, as this detail doesn't show in photos.