Short answer - yes, you just pry the ends of each side of the trucks apart and insert the replacement wheel sets, one axle and then the other.
As far as axle length goes, if the standard replacement sets such as Intermountain, Kadee, Proto 2000, ExactRail etc. are too loose, I suggest you get a Reboxx sample set which includes a range of axle length wheel sets which are color coded. You can insert each, one at a time to find which axle length fits best. Once you have identified the best length, you can order a set from Reboxx for the model(s) needed.
Rio Grande. The Action Road - Focus 1977-1983
Guessing your talking about HO scale?
I usually remove the truck, and then each axle, use a Micro Mark truck tuner to clean out the pocket in the truck where the needle end of the axle fits, install the new wheel sets, and give them a spin with my finger and see how the work.
You also should have a gauge to check the new wheel sets.
I've also done this with talgo style trucks ( the coupler is attached to the truck) with decent results.
Size, as far as wheel diameter, is measure from the tread, the part that sits on the rail. Typical sizes are 36" and 33". If you have trucks that the average wheel set doesn't fit, you'll need a caliper to measure the length, from point to point.
If you want to change from talgo style to a seperate truck and coupler, that's another procedure. Get the trucks your going to use, remove the talgo truck, fill in the hole in the bolster, redrill it for the new truck screws, and then body mount the coupler. You should have a coupler height gauge to get it right. Kadee makes a nice one.
Without getting into specific details, that's about the jist of it. If you want more detailed answers and how-to info, tell us in more detail what you have.
Mike.
My You Tube
I have searched the forum for this so I guess I am the only one who doesn't know the answer. To change the wheel sets on my cars do I just pry the trucks apart? I have some cars with wheel spacing that is totally out of spec that I want to replace but I don't want to tear anything up doing it. Another thing that would be helpful is exactly where to take the measurement to see what size wheels I have. I have never tried working on my trucks before which is probably why many of my cars like to derail often. Most of my rolling stock is not the most expensive to be found! Any help would be greatly appreciated. I find myself learing an awful from this forum.
Thanks,
Dave