Being fairly new to O Gauge I have a question -- What is the difference between 2 rail and 3 rail ( besides the obvious number of rails ) and will they run on each other?? Thanks.
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3 Rail gets it's power (AC) from the center (third) rail. 2 rail operates like HO, N, etc. The 2 rail O models are the same size as 3 rail O and in most cases are actually models converted from 3 rail. For more info., Google is your friend! 
With 3-rail O-gauge, 1/48 scale is more of a "suggestion" than a mandate. With 2-rail O-scale, it's the other way around.
2-rail and 3-rail do not necessarily run well on each others tracks without a change of wheel sets (not to mention electrical pickup on locomotives).
Per MikeH, Google is your friend. 
A few years ago I heard a joke, from a 2-rail scale person, no less. He said, "A 2-rail scale person, running on a bare plywood base, feels superior to a 3-rail person running on completed scenery."
Maybe Google brought him here?
Easy enough to ramble on....
Are you going strictly 2r or 2rs?.
Mixing any 3r or shedding the center rail? Mixing look at track brands with squared railhead and 3 isolated rails. That would allow you a good mix and going ac or dc, 2r or 3r, at the flip of a toggle or two.
2rail will have smaller flanges on wheels and skinnier wheels, flat rail heads. Likely will use body mount KD couplers vs talgo style (coupler attached to trucks)
Being clean is a 2r must. Don't skimp on track.
Weighting of loco, cars, etc. usually a smart move for reliability (visiting NMRA yet? Gauge tool, coupler height tool, and weighting charts are your new friends )
3rail usually has an oversized "lobsterclaw" coupler, larger "pizza cutter" flanges for less derailment on much tighter "spaggetti bowl" turns than most 2r. Modern 3r use "fast angle wheels"; the tread of wheels is angled greatly, reducing drag with a smaller contact to rails, and acting as a natural diffential gear in curves; leaning to the inside in curves (a prototype feature exaggerated to work well here.)
Converting cars from 3r to 2r requires plastic wheels or insulation of left metal wheel from right to prevent shorts.
2r on 3r for cars isn't an issue if the flanges can manage the track. They won't trigger 3r isolated track accessories/anti derail.
Lighted cars need pickups added 2r to 3r. ..3r to 2r needs isolated wheels and wheel/hub/axle contacts or rail wipers, battery/led etc .
2r does have some oversized wheel flanges and widths (measurements of flange & rail height is the "code"; e.g. code-80 code-100), the larger codes stand a chance on 3r track. The traditional round tube track of 3r doesn't like small flanges, but GarGraves, Ross, MTH, Lionel, Atlas, and others use either a near square rail head or 'proper' solid rail and can perform very well with some 2r wheel sets at least. (I don't know Rivarossi code)
2r track laying is more sensitive to mistakes because of smaller codes.
Two rail often uses dc voltage. But ac and dc trains exist in both 2r&3r, so beware. You can run an ac motor on dc 99% of the time, but a dc motor on ac will usually toast quickly.(magnets for motor field (vs field coil) normally mean a dc only motor for us) (that missing 1% the dc power still won't wreck an ac motor, it just may not work.
Rectified ac to dc is pretty simple, but not always best for some command control systems that like a different sine wave shape. Pure vs chopped, square, freq. mod, etc.
Non command mode and conventional AC trains/tenders may blow the whistle/horn/bell nonstop if run on dc. (the horn trigger is an offset ac wave, e.g. +10v for lead half, back to 0v, then to -13v for the other ac half, gives a -3v difference in wave size seen as -3v dc by a dc only relay .. -3v offset would trigger a bell, +3v triggers the whistle (old trains see either offset and just have horn/whistle, no bell.)
The wiring of 2r track may be much more involved if you go beyond loops. Reverse loops (teardrop turnaround) needs track detection and relays to reverse the power leg polarity or the loco shorts as it touches both of the power blocks when going over them, or it suddenly reverses, then maybe fwd,back,fwd back etc. if it managed to not short long enough to trip breaker or fuse. The relays keep the correct powerleg on tap when track meets itself in opposing directions.
I think maybe smaller scales &/or ac/dc command by "somebody"(?) has overcome that though.???
Converting most 3r engines to 2r is a lot of work needing all isolated wheels . MTH sells some "convertible" 2r/3r locos that are pretty simple to change (remove center pickup and throw a microswitch) DYI converting 2r locos to 3r is much easier, tie the wheel wires together and add a pickup... if the flanges can manage the track you're rollin'.
Whenever I think about this; another type of 3r comes to mind, though Euro & vintage (Märklin) It used studs on the ties vs a center rail and appeared much more like 2r. Also, vintage "outside 3rd rail" ( a great craftsmen nitch imo, mostly scale but the craftsman aspect means anything goes). There a short 'guard rail' run outside the track and a bar/shoe/etc. extends laterally, that rides it vs a center rail.
Dewman51 posted:Being fairly new to O Gauge I have a question -- What is the difference between 2 rail and 3 rail ( besides the obvious number of rails ) and will they run on each other?? Thanks.
You’ll get more responses if you post this on the correct forum. This is the software technical forum. Welcome and enjoy.
