Going from 2 to 3 rail is not tough. It is very simple. All you need to do is add center pickups and rewire the loco to pick up off the center and one side instead of off one side on the loco and one side on the tender.
What is very difficult is getting a 2 rail loco to go around typical 3 rail curves. 48 inch radius, (096) is considered a minimum for small to medium steam and 54 to 60 inch radius is most often recommended.
Most mfg make a lot of compromises to get a medium to large steamer to go around 3 rail curves. As mentioned above, they most often lengthen the pilot truck and the pilot deck so the pilot truck swings around the cylinder ends. they often have very little if any frame under the cab so the trailing truck can swing really far. They almost always have blind drivers except the two ends. These compromises often mean the smokebox and the firebox are longer than scale prototype. also the drive wheel spacing will often be wider than scale to accomodate the larger flanges.
Now one thing in favor of scale locos is that they almost always are built with articulated side rods. these are the big rods that connect the drivers to each other. They are hinged at each driver on the prototype and also on most 2 rail locos done specifically for 2 rail. This allows the two end drivers to slide slightly sideways in the SAME direction on a curve. All 3 rail locos I have seen do have this feature. they don't need it with blind drivers. They all have solid, one-piece side rods, so when the lead driver goes into a curve, it pushes the rear driver in the opposite direction if all wheels were flanged. For example I had a 2-rail version of a 2-10-4 built by a major 3 rail mfg to review for a magazine. It was a nice model. the 3-rail version would run on 072 or some such according to the mfg. The two rail version would not go around 60 inch or O120, it had solid siderods and flanges on all drivers. My layout has 52 inch minimum radius and it would not go around my curves. I laid some temporary track and found it go around about 72 inch radius (0144). the drivers had plenty of sideplay, but the siderods would let them adjust to curves. I also owned an 0-8-0 switcher that had the same problem, it stalled on 48 inch curves but was happy with bigger stuff. It was the diecast PRR 080 that came out some years ago.
