Brian,
It seems most N scale locomotive manufacturers undergauge their wheelsets. Instead of purchasing a unitasker tool, I just get between the back of the wheel and the truck with a small screwdriver. A little twist and you can move the wheelset out. After doing enough of these you should get the feel for how much to twist. Don't worry about going too far, because you can squeeze the sideframe and move the wheel back in, too. The only tool that you need, besides the screwdriver, is an NMRA gauge.
Help this helps,
Dave Foxx