An already powered frog for a Rivarossi Big Boy???

What do you mean you wired the frogs for 'all power'?

On Atlas turnouts, the frogs are isolated, but the rails beyond the frogs are wired to have power on at all times. An engine as large as the BB should have no problems with Atlas turnouts as-is. Unwire the frogs, and see what happens.

If you insist on wiring the frogs, make sure that they are wired so that their connections to the running rails change when the switch is thrown

- to lefthand rail when thrown to the right, to the righthand rail when thrown to the left.

Also check that you have insulated railjoiners in the proper places.

HTH

Shawn, First check the gauge of the turnouts and your locomotive. Some times an adjustment of something slightly out of gauge solves all problems. I did have one Atlas turnout that lifted the wheels of my locomotives just enough at the frog to stall most everything at slow speeds. A quick shot with a file between the rails at the frog took care of the problem. If your Big Boys are older models with deep flanges that might be a possibility especially if your other locomotives are running through with no problem. Even after checking the gauge and making various adjustments or modifications to turnouts take an un sprung freight truck and hold your finger on it very lightly and move it back and forth through the turnout. You can feel even the very slightest hump, bump, dip or snag that might need attention if you have a loco that's fussy at turnouts. Other than that the only thing I can think of is that you are getting a short somehow. Bruce