As HW says, it depends. You need flat-top rail like Ross, Atlas, MTH ScaleTrax, or Gargraves.
The modern higher-end brass and Lionel's new replacement wheel sets tend to have the .145 wheel treads, so if your track has slightly wide gauge (like Gargraves Flex), you may get "drop in" derailments with the gauge. Atlas 2-rail rolling stock, MTH scale wheels, and Intermountain wheel sets (Weaver replacements) seem to work well.
Turnouts create another issue, though. The turnout can't be larger than a #5 (11-degree Ross) as the frog gets longer with #6 and #8 turnouts and the wheel drops into it. We modified a turnout with a point within the frog to compensate for this. Curve-replacement turnouts (O-72, etc.) will be problematic, especially during backing moves because the arc of the curve extends through the frog and the guard rails are ineffective with scale wheels.
Curve requirements depend on the length of the car, coupler swing, and the distance between the end sill and the truck bolster. Atlas 89-foot Trailer flats can negotiate 36" radius (O-72) curves because it has long-shank couplers (I replaced the plastic Atlas ones with metal Kadees). A passenger car with long-shank Kadees can make tighter curves than one with regular-shank Kadees. You also have to be cautious regarding which cars are coupled to each other so their end sills have roughly the same swing-out on curves to prevent one car from derailing another. All that said, when it comes to model railroad curves, bigger is better.
Car weight is another issue. You want your heavier cars up front or a bunch of properly-weighted short cars to get more flanges toward the front . The NMRA weighting recommendation is 5 ounces plus one ounce per actual inch of car length, so a 40-foot (10-inch) car should weigh in at 15 ounces. We keep a postal scale and stick-on weights at the club as we're on a program to get all of the club cars properly ballasted (we put the kids on this one so it's taking a little longer).
You're also going have to get used to "problematic" cars that seem right, but randomly derail. That's the price you pay for living in both worlds. Sometimes making modifications to the trucks are required. On some of my Atlas cars, I take out the secondary crossbar to allow the trucks to float a bit.
Despite some negatives, the positives are:
- The scale-wheeled rolling stock rolls better. I've tested this and it's very consistent.
- The equipment looks better as it's closer coupled.
- No traction tires on locomotives!
- You'll find all of those bad spots in your layout. These are the ones that cause minor derailments with scale wheels but, in time, cause disastrous derailments with hi-rail equipment. Yes, the club members rib me about scale wheels, but like that our track is getting cleaned up.
- Scale wheels are quieter than hi-rail wheels. This is probably related to the material and construction of the wheels.
- You can find some really nice 2-rail cars out on eBay and from others that aren't available in 3-rail. Yes, they're expensive, but there's some nice stuff out there.
