How can I tell the difference between 2 Rail and 3 Rail Trucks&Wheelsets?

Look at the flanges would be my first thought.  3-rail flanges are bigger.

Use an ohm meter to check for continuity between wheels, if the meter buzzes it's 3-rail.

The deeper flanges are for three rail running, the less deep flanges are for two rail operations.   The wheel sets on the deck are two rail, the ones on the track are three rail.   On a two rail truck, one wheel on each axle is insulated from the axle and those insulated wheels are on the same side of the truck. One or both trucks may be insulated from the frame of the car/engine.  That allows one pair of wheel sets(one truck) to draw electricity from the positive rail and one set to draw electricity from the negative rail.

You are not going to do any damage running two rail wheel sets on three rail track.  They may not span the gaps in track switches, but they can be used.  However, three rail wheel sets on two rail track will cause a short circuit just as quickly as laying a nail or pair of pliers across the two rails.

John in Lansing, ILL

Bob Delbridge posted:

Look at the flanges would be my first thought.  3-rail flanges are bigger.

Use an ohm meter to check for continuity between wheels, if the meter buzzes it's 3-rail.

What setting do I put the ohm meter on to check this?Pardon me,but I am fairly new to using an ohm meter!!!

Hello,So I can just swap out the wheels and still use the 2 rail trucks then???Pardon my ignorance,that is why I am here asking for help.-Kenny

Kenny,  With larger diameter flanges you could have a clearance problem between the top of the flange and the bottom of the car when you replace two rail wheelsets with three rail wheelsets.  John in Lansing, ILL

you put the meter on the diode function near bottom and to the right near bottom of selector switch on your meter ! 

I have meters everywhere.  None have a diode function.

Look for an "ohms" setting.  Sometimes it is a Greek Ω. There are usually a bunch of scales - just choose one.  Hold the probes together, and watch the needle (or the digits) go to zero.  That means a dead short.  Then do that to your wheelsets.  No needle movement means insulated.

Some scale wheelsets are uninsulated.  Don't throw them away - some of us use them.  

To directly answer your question: on the left is an older scale truck.  On the right, with darker wheelsets, is a “Hi-Rail” truck.  You can readily see the difference.  With true tinplate wheels, the flange is even deeper.  You should have no trouble selecting if you keep this picture in mind.

Often, as with Weaver trucks, the only difference between a 2- and 3-rail truck was the wheel sets. They interchanged easily.

Both the flange depth and the tread width figures into 2/3-rail wheel differences. The flange is the more critical, but some older "2-rail" wheels had deep flanges but narrow treads. they may work in 3RO environment, or not. Case by case.

The larger flanges may rub the underside of a so-called "2-rail" car. This may be no problem if occasionally, or it may have to be fixed with spacers at the truck mount (which, if too thick, make the car look real funny). Case by case.

The insulation of the 2R wheels is irrelevant to the 3R system - except in relation to insulated track sections for accessories and/or signaling - but that is another conversation.

Thanks everyone for your replies. I think I am starting to understand the differences now. I have a bunch of OLD wheelsets from the 50's, I will post pictures of them later and maybe you guys can tell me if they are 2 or 3 rail

did u get the pic I  sent you of the meter shows exactly where to dial the selector for diode continuity  for your axle of the wheel sets 3 rail will beep 2 rail will not beep due to isolation between axle and wheels!

bob2 posted:

To directly answer your question: on the left is an older scale truck. 

Bob2,

That one looks like an older Athearn metal truck, one of my favorites; in any of its four forms, but especially the Andrews. Too bad that A did not make a Fox truck.

Fox trucks are easy, unless you need them sprung.  I made a master once, but it got lost at the foundry.

Thomas made good die cast Fox trucks, and I have a pair of sand cast bronze, source unknown

The only Fox trucks that I have seen are the tender trucks on an O scale AHM steamer and this pair (pictured below) by an unknown manufacturer that I have: