Needed advice

 The problem with the hi-rail wheels is not so much the height of the rail vs flange height -- I think 148 should be fine -- but rather the problems at turnouts:  Because the back-to-back of the wheelsets is slightly less, the flanges will hit the guardrails and frogs of turnouts.

Also, with MTH there is sometimes a disconnect between what the catalogue says and what the model really is.  To me, a loco advertised as 3-2 capable but not offered in a scale version would set off an alarm bell:  I'd independently verify the loco in question really is 3-2.......

Best rgds, SZ

I can only add that it does take some effort to make even a modern 3 rail 3/2 model, run on 2 rail track. If someone wanted the feature of being able to run on both, I would suggest ordering the scale wheeled model. That would not be great for running on tubular track though I've heard. If there was not a scale wheeled version of the steam you wanted, I would first look into a RTR 2 rail version somewhere else.

I have heard that some recent 3 rail MTH 3/2 versions are set-up to run on 2 rail easily. I can't confirm that. As far as the older models I have got my hands on, it's not that simple.

 There are other posters here that maybe better at offering advice on this. I try just to buy scale wheeled versions for my 2 rail layout.

Todd, I'd say stick to the locos that come with the scale wheels from the factory. Conversions are a pain in the butt!

In case you are interested, I'm slowly selling my MTH 2 rail locos, these include a SP daylight GS4 and some ES44s and Sd70s.

In the last month, I have converted several large MTH and Lionel locomotives from 3 rail to 2 rail for a group building a large 2  rail O scale railroad up in New England.  This group included an MTH Erie Triplex and a DM&IR Yellowstone.  Converting any of these 3 rail locos to 2 rail properly requires a good metal lathe, some special tooling that can be made on the lathe, accurate machinist's measuring tools and the knowledge to use them.   You generally have to re-machine the hi rail drivers and install new scale tires insulated for 2 rail.  This is not a five minute job and can become tedious.

I do not think much of MTH 3/2 locos for a 2 rail modeler.  Maybe their newer ones have paid more attention to scale driver dimensions.  MTH also does some odd things to make these models 3/2 like split driver axles and insulated crankpins and axle bearings to avoid insulation at the tires.

Joe

Some pictures of a 3/2 Premier Russian decapod.  You have to remove the third rail pick-ups from the tender.   There is a  3/2 switch on the tender and a Polarity switch.  Isolation left to right is done as Joe has mentioned.  You would still have to deal with the larger flanges, that are 3 rail.  

Insulated frame and axle bushing

Power collector on the front tender trucks. One tender truck is left rail the other truck is right rail. The third rail collectors are easily removed.

Rear truck common collector.  Third rail pick-up has to be removed.

There are two switches for two rail operation.

All other drivers/wheel sets are isolated left from right.

-Thanks everybody for the advice. I have enough "conversion work" going on already with my outdoor projects (G scale/Battery Power with enhanced steam effects)...So I don't really want to tackle converting older stuff to PS3 and 2 rail, at least not right now. I will stick to scale wheels only and scale track. Since my roster and collection of 2 rail O scale will be very limited in size I do agree that it's probably best to just get it with scale wheels or don't buy it (speaking steam as the diesels look much easier to convert given the kits).  Once I get up and running I will post on YouTube and give you guys a shout out. Thanks again, it's much appreciated....Todd